The biggest in terms of volume would probably be a Matrioshka brain.

A Matrioshka brain ^[1] is a collection of multiple Concentric Dyson Spheres which make use of different wavelengths of light.

Here's the list from Wikipedia's article on Megastructures ^[2] where I found it:

• The Alderson disk ^[3] is a theoretical structure in the shape of a disk, where the outer radius is equivalent to the orbit of Mars or Jupiter and the thickness is several thousand miles. A civilization could live on either side, held by the gravity of the disk and still receive sunlight from a star bobbing up and down in the middle of the disk.
• A Dyson sphere ^[4] (also known as a Dyson Shell) refers to a structure or mass of orbiting objects that completely surrounds a star to make full use of its solar energy.
• Larry Niven's Ringworld ^[5] is an artificial ring with a radius roughly equal to the radius of the Earth's orbit. A star is present in the center and the ring spins to provide artificial gravity.
• A Matrioshka brain ^[6] is a collection of multiple Concentric Dyson Spheres which make use of different wavelengths of light.
• A Stellar engine ^[7] either uses the temperature difference between a star and interstellar space to extract energy or serves as a Shkadov thruster.
• A Shkadov thruster ^[8] accelerates an entire star through space by selectively reflecting or absorbing light on one side of it.
• Topopolis ^[9] (also known as Cosmic Spaghetti) is a large tube that rotates to provide artificial gravity.

A couple of the others that move entire stars around would probably be considered bigger in terms of mass if you count the mass of the star.

In Greg Egan's Diaspora, the manufactured objects at the end of the characters' journey are inconceivably vast in terms of our universe. Each sub-atomic particle contains an entire universe, each sub-atomic particle of which contains an entire universe, then iterate millions of times until you get to our universe.

Nothing else in SF comes within trillions of orders of magnitude of being as large.

Isaac Asimov's Multivac.

In "The Final Question," Asimov refers to the last version of Multivac as the known universe. It finally becomes a god when it answers the question: Can entropy be reversed?

A device that's surrounding a star is fun but why not use those stars to build something? Fry did it!

How about the Great Attractor created by the Xeelee in Stephen Baxter's Ring? Being an artificial structure of galactic super-cluster proportions, it should at least be on the list.

In Stargate Atlantis the star gate's on Earth and Atlantis create a wormhole three million light years in length. It is a single engineered object when instantiated. Of course three million light years is its perceived external length; presumably it is much shorter when travelling through it.

The wormhole in Stargate Universe at the start of the series is much longer (but I don'r recall how long).

Not as big as an universe, but the "Hitchhiker's Guide to the Galaxy"'s planet-builders are impressive and fascinating.

Unfortunately, the planets would be naturally occurring, but, who knows at the end...

Schlock Mercenary ^[1] has a power generator built from the Galactic Core. It was originally built as a sort of trojan-horse (the beings who designed it wanted our galaxy destroyed, the beings who built it were misled into thinking it was just a generator). It was stopped from being a bomb and kept as just a generator by an alliance of pretty much every sapient around, along with the formation of the largest AI around.

Ringworld ^[1] However, I know you guys can do better! It is roughly 300 million miles in circumference and about 1 AU ^[2] in radius (i.e., 93 million miles). It is composed of material with a tensile strength close to that of the strong nuclear force. It is roughly as massive as all the planets in a star system.

In The Galaxy Railways ^[1] there is a galactic size interstellar railway system. (This is the same universe as the Harlock Saga ^[2], and Galaxy Express 999 ^[3])

In "Bill, The Galactic Hero", ships are powered by a "Bloater Drive" which causes the ship to expand until it's as big as the galaxy. When the ships are that big, the individual atoms are so far apart that no one notices the presence of the ship. The ship is then reduced back to its original size at a different location.

In Time Keeps on Slippin ^[1], an episode of Futurama, Fry uses a Bad-Ass Gravity Pump ^[2] to move a whole mess of stars to spell out a message to Leela. The message reads:

"I Love You, Leela."

In Peter Watt's Blindsight, the alien artifact Rorschach is incredibly massive (although shadowed by a nearby planet), but there are indications that it is constantly growing, possibly beyond plantery size.

In Alastair Reynold's Pushing Ice, the structure in which the ships are trapped is engineered and at the end it is indicated that its size is immense (possibly light hours in scale).

In the Star Trek universe, it’s surely the Borg Collective. It spans a significant proportion of our galaxy.

In Dan Simmons' Endymion books, the Ousters were growing modified trees in orbit around a star, intending to eventually enclose it completely in a living Dyson sphere. I remember the phrase "Leaves the size of continents". Though they did not do so in the book, their plan was to eventually do this with other stars, and maybe connect them, to fill the space between with life. I give them credit for thinking big.

Also, in Carl Sagan's Contact,

at the very end of the book it is mentioned that a message is hidden in the digits of PI, which suggests that all of space/time was created by something...

In Terry Pratchett's Strata, the protagonists' job is to build habitable planets with fake histories, such as fossils for beings that never actually existed. There seems to be an irresistible urge for such builders to include little impossibilities as signatures. The characters do learn more during the book, and...

At the end of the book, the protagonists find one of those little indications that their Universe was built by people with the same "signature" habit.

In Stargate universe, Rush discovered that the Destiny was launched to investigate a hidden message written onto the cosmic microwave background. Presumably some intelligence that existed prior to our universe's existence was able to emplace this message using undreamt of physics. Unfortunately the series terminates, so we will never find out who (or what) created it for what purpose. But it does span the entire universe!

In mass this might not be very big, but in terms of the scale of distance it covers, it's HUGE - the stargate's in the stargate series. A series of wormhole transports that span not only this galaxy, but connect over to another (Andromeda) galaxy.

How about the "Fleet of Worlds" - the home planets of the Pierson's Puppeteer race. Consisting of the actual homeworld, called "Hearth", and its agricultural worlds, arranged in a pentagonal Klemperer rosette. and accelerated to 80% of light speed by inertialess drives.

I voted for the dyson sphere as being the biggest realistic. If you want something that's not as big but maybe more realistic - then the culture ships from the iain banks books. Ships that are miles long, housing millions of people, other ships and capable of building other ships.

Perhaps not the biggest, but the computers in David Zindells A Requiem for Homo Sapiens trilogy, The Broken God ^[1] being the first, the computers are truly vast covering 300k planets with each planet computer associated with a star to power it. The plot deals with people being vastened from normal humans into each of these computers, and some of the vast computer sentient entities being de-vastened into a childs toy, after being destroyed. A trully excellent series of books worth a read, about the ascendency of man, written in Zindells poetic language.

In the Cyberiad from Stanisław Lem someone constructs a giant robot out of nearly all the matter in the Universe. The whole story is quite tongue-in-cheek, but it might still qualify.

The structure simply known as The City or the Megastructure in Tsutomu Nihei's Blame! ^[1] appears to outclass everything else here.

It's a classically engineered structure (i.e. no quantum "there's an entire universe inside!" cop-outs) that extends roughly spherically outwards from the center of the solar system to a point at or beyond the orbit of Jupiter. It's constantly expanding, being built by out-of-control AIs. It's multi-leveled ("the megastructure" nominally refers to the constructs separating the levels but is also shorthand for the whole thing sometimes) and so the theoretical "habitable volume" is enormous, although in practice few areas have oxygen, or gravity, or lack of killer androids, or etc.

Nominally this makes it a Dyson sphere, or rather a series of Dyson spheres, but I'm not aware of any other work of fiction that portrays a Dyson sphere this large. It also has some elements of a Matrioshka brain, as some kind of computer network appears to run throughout it.

I think we all are forgetting "The Death Star" here and ultimate weapon as big as a few hundred suns.

Allen Steele ^[1]'s book Hex ^[2] describes a Dyson sphere, but made of hexagons, 1000 miles (K? I forget, but 1000 of the units) long, 100 miles wide, each as a discrete habitat.

Ya, I know Dyson spheres have been mentioned, but this is an interesting example of it. 36 Trillion biopods (each arm of each hex).

Other Dyson Sphere example might be in Roger Mcbride Allen ^[3]'s Hunted Earth series (alas only two books, but great ones!)

• Ring of Charon ^[4]
• Shattered Sphere ^[5]

How about The Stone from Greg Bear's EON. The description of it is that it was endless, bigger on the inside than the outside; a hollowed-out asteroid connected TARDIS-fashion to The Way, an cylindrical artificial space a dozen kilometers in diameter and infinite length.

In the sequel, Eternity, it is revealed that the Way is not actually infinite; it ends by opening up into a small universe (finite but unbounded). The length of the Way is difficult to calculate from the book, but since it takes the Geshels at least several thousand years of subjective time to travel it at very nearly the speed of light, a rough estimate of a billion light-years would be very conservative.

In the Perry Rhodan Multiverse we have

1. The Loolandre ^[1] is the heart of the endless armada ^[2] with a diameter of at least four light years
2. The Tiefenland ^[3] is a discworld with a diameter of one light year (and a transmitter dome every light second)

Isn't there a space-opera series where an (obviously) disappeared alien race literally carved a galaxy ? I even remember there are some kind of tourists visiting this galaxy.In fact, I remember, it's not a galaxy, it's a chain of stars, linked each one to the other by ... a typical Ancient Disappeared Race means. According to timday's excellent comment, it's the kiimt homeworld in Night's Dawn hyper-huge trilogy of Peter F. Hamilton.

The Magog World Ship is quite big, i can't find any facts about its size, but it's a solar system with at least 11 planets..

Starting in the second season of Lexx, Mantrid created a von Neumann probe drone of his own arm. Eventually Mantrid is tricked into summoning all of the Mantrid drones, which at this point had consumed all of the matter in Mantrid's original universe, to one place.

The Mantrid drones collected all of the matter in a universe and artificially created a big crunch when Mantrid summoned them to the same location.

One could argue that the creation of a parallel universe would satisfy the conditions of the OP's question.

• The torbeast in Neal Asher's Cowl created parallel timelines.
• Brandon Mull's Beyonders – engineered universe
• Alistair Reynolds' Absolution Gap – parallel universe that has been ultimately destroyed by rogue terraforming machines (not an engineered universe, however any terraforming system that can take down a universe would have to be fairly large)

and others...

For the answer of mass I'm going with the transmitter stations in star trek voyager all together there where a couple hundred but each one was powered by a black hole if you but them together you have allot of mass sents they where artificially created

Borg's Hypercube.

In Star Trek: Alien Spotlight - Borg comics, Picard located a point in Spacetime which was a whirlpool of Temporal Energy on which many many future events were balanced (According to calculations of Data). After jumping through it, Enterprise-E encountered Borg from the future. They found Borg's Hypercube which was very-very big in front of which Enterprise-E looked like ant.

Cybertron.

Its transformed and engineered body of Primus. Its used to be home planet of Transformers robot. Its size is significantly bigger than Earth. See its emerging body from space bridge in front of Earth as displayed in Transformers: Dark of the Moon movie: