AFAIK, the largest structure in fiction would be the Ring, in Stephen Baxter's "Ring" novel. Composed of cosmic strings, I believe it was something like a few million lightyears in diameter, and so massive it was identified in the novel as being what real-world astronomers currently call the "the Great Attractor": the greatest known concentration of mass in the universe, drawing galactic superclusters in its direction with its gravitation.
Originally posted by Galan007Wrong on both accounts.
The Quantum Mechanics and Monitors have built the largest structures I'm aware of. The Quantum Mechanics built Heaven's Ladder. Monitor NANO-tech was the size of a city, and it's computers held INFINITE storage capacity.
Juntai's Penis is the correct answer. Largest structure in fiction.