In one of the anthologies for Larry Niven (either N-Space or Playground of the Mind), he describes a Ringworld that induces magnetic effects on its star to shoot out a jet of plasma, using that to propel the whole system.
While trying to find an online reference for this, apparently in the last Ringworld novel they hook a hyperdrive up to this, making it the largest "ship" I know. (I use "I know" advisedly... I'm sure somebody somewhere has turned a multiverse into some sort of ship for some reason or other.)
In the Cities in Flight series by James Blish, the planet "He" gets about quite well, though that is dwarfed in every measurement by the Ringworld.
what would V'ger count as? If we would count it as a ship would we count its energy field in it size or just the support area directly around the probe itself? Based on "Trek Wikia" the novel put it at about 78km for the vessel itself. Pretty much one of the most powerful space moving vessels conceived.
