I'm not a builder by any means (I'm a "caver") but this is the largest base I've built, measuring 70x88 blocks and up to 36 blocks high and containing about 18,000 blocks:
A 3D isometric rendering made with MCMap:
I took these screenshots in a copy, using MCEdit to remove the top to make the cutaway views. The giant trees in the front are from my own mod (not player-built):
With the top removed:
On the left is the storage area for all the resources I collect while caving (I accumulated about 1.5 million resources by the time I stopped playing ion this world, double-stacking chests to add more storage space); on the right side is my Nether portal, enchanting/brewing room (not really used after the early-game), bedroom, and workroom (commonly used supplies):
A self-built village, entirely just for fun as I didn't trade with them in this world (I do trade for diamond gear in my first world but I don;t have any "trading halls", just villagers roaming around a naturally generated village, aside from the ones that I trade with, which are locked up inside houses):
Farm area; I really only use the potato farm at the bottom (the largest one) and the occasional sugarcane for maps:
Tree farm; I use jungle wood to make torches while caving, using about half a stack per play session, otherwise I just use whatever trees are around, usually oak, for wood in my bases:
Not sure if this counts, but I'm currently working on my largest project by far, I've been building something of an adventure map for a few years now and I'm probably about half way through the city right now. Well maybe more like a quarter since I haven't done any interiors yet...
It's starting to come together, slowly, and then when I'm done building all I'll have left is to tie it all together with a story. easy peasy /s
It's probably taken so long since I keep going back to redesign old builds, recently I completely rebuilt the castle in the back there and I'm still not sure I'm happy with it. Might be in for more tweaking later on.