At the moment it looks like there is three sizes of theaters, 64 x 64 segments (original Korea, Balkans, Nevada, Taiwan), 128 x 128 segments (Afghanistan, Europe, Iran, Korea 128, ODS and Vietnam) and then finally there is the 256 x 256 segment theater size, to which so far nobody has released any theaters.
At the moment most common seems to be 128 as the “major” theaters are done using this size.
We have to understand that the size of the 256 segment theater is just mind blowing. As the default size is 64 and 128 theaters are “huge” compared to that, the 128's are little boy-scouts compared to the big bad 256 segment theater. The TGA image to create the TE/Campaign map is on 8096 x 8096 resolution and the size is about 65mb for that file. Yeah, think about theater download sizes, just a background image. Tacedit job for 256 theaters is pretty easy, just change the theater size and that's it, no big deal.
Here is in-game-campaign screenshot (merged) of the whole theater area. There was over 18000 objectives on that campaign, when I flew one mission OCA strike I detected nothing out of the ordinary. FPS was just the same as when flying 64 segment theater.