10 Largest Megastructures In Star Trek

Although we never learned the exact size of Starbase Yorktown from Star Trek Beyond, it is undoubtedly the largest structure Starfleet ever constructed. From comparing it to the Kelvin Enterprise, we can infer that it was well over a dozen kilometres long.

Starbase Yorktown likely only existed in the alternate Kelvin timeline, which is quite infamous for upscaling Starfleet tech. The Kelvin Enterprise was also significantly larger than the Enterprise from TOS. This deep-space gigantism was mostly a product of Starfleet's increased focus on defense and technological development after Nero's attack and the subsequent changes to the timeline.

Yorktown was, at the time of its construction, the most advanced starbase in the Federation. Its massive internal space contained multiple cities, gardens, and even a complete shipyard all intertwined in a network of huge rings and arms, and enclosed in a snowglobe-like translucent sphere. It's a bit odd that Starfleet would build such a massive and advanced station in the middle of nowhere, on the edge of charted space (right next to a big, scary unexplored nebula, no less). The rationale given in Beyond was that Starfleet chose to place it so far out from any Federation worlds to avoid showing favouritism to any of its member species. 

To compensate, Yorktown was equipped with heavy defensive turrets and satellites, but this turned out to be a major security risk, as Krall took advantage of Yorktowns isolation to wage a massive attack, hoping to kill all of the millions of residents of the base and use its weaponry and manufacturing abilities to kickstart his invasion of the Federation.