The current largest flyable ship in Star Citizen is the Starfarer, a standard fuel transporter vessel, and is the feature of the video below by level lead Matthew Trevelyan Johns. Presented in glorious 4K, it represents yet another way to appreciate the superb technical achievement Star Citizen is, as well as the game's ship design.
If the Starfarer looks big to you, it's nothing compared to the Bengal Carrier, a monster of a ship that appears to be roughly 100 times bigger in this chart. Of course, due it's massive size, it'll likely be awhile before the Bengal is playable.