Hmm, there used to be a super-ship hype a while ago, with super juggernauts several times the size of the Sathanas. The community has mostly moved away from these: they are rather difficult make them fun for the player to interact with, tricky to move around the battlefield, take a lot of resources and take a lot of work to create and make them look good. I know the original Inferno used to have such massive ships.
I think Inferno still uses massive super-bombers, that we'll see in action again later this year.
The largest installation I can think of is the
GTI Whitehall, I not sure if it's been used anywhere. Again, Inferno might have had massive installations as well, but someone that actually played it will have to confirm that

Outside Freespace, The Babylon Project had, well, Babylon 5.

Blue Planet uses a big asteroid in one of its missions, as well as a lunar landscape. Out of the FS universe, Diaspora also used a few big rocks.