The largest thing I made was also an underwater house. I did most of the work, although Tpcool expanded on his apartment a bit.
It contains:
An entrance
A chest to put shit we don't want in
An incinerator to burn the shit we don't want in the aforementioned chest
A (very small) farm
A (very slow) tree farm
6 fully furnished apartments, complete with bed, chest, furnace, crafting table, painting, two bookshelves and an exit to both the hallway and to the water
A basement with walls made of glowstone, and an incomplete roof made of bricks
A large room with a Nether portal
And then I think we're going to have a cake soon
Also mine and Tpcool's rooms are connected via secret door. Of course, I can literally lock him out with the push of a button.
What makes this more impressive:
We did it on the XBLA version.
That's Beta 1.6.6 with a couple bug fixes.