Maybe Bree villages are the largest? the gondorian and easterling towns are probably equally sized.

I contacted members of the official server and some are saying bree

Does the ring of Isengard count?

I dont think so, because that always spawns in around the same place

But that would make the bree villages unviable too?

No because they are naturally generating

They dont always spawn in the same place

Bree-villages always are in the same place right?

I would say the largest structure is (if the Isengard ring counts) a mountain, since they are counted as structures, right?

Wait bree villages are set spawns? and no, I decided not to count mountains, simply because that would be unfair

@Delirium013 Yes, the four major Bree-Towns are fixed structures.