This is of course a trade off that's not really in the poll.
For example, all things being equal, of course I'd like the land to be bigger. But if being BIGGER than Cyrodill means that it will have less detail or interesting places then no.
As a case in point, although I really haven't gotten into New Vegas yet, if you look on those boards there is a lot of discussion about how New Vegas is about as big or bigger than Fallout 3, but not nearly as good for exploring. Because there is nothing interesting in those places. And large regions are inaccessible.
So I wouldn't want it to be bigger than Cyrodiil if it was just full of uninteresting and unique places. (and yes Cyrodiil has many many interesting and unique places, so let's not go there please).
... 3 environments at most? snow region, forest region, oblivion region.
Jeepers. Cyrodill: Snowy, Lush Grassland (skingrad), Forrest, Dry Grassland (Anvil), Marshes (Leyawiin), hills (eg north of the IC), mountains (far west and east) and if you want to count it Shivering Isles; Morrowind: swamp, grass land, lots and lots of barren rocks, and rocks with dust storms. Sorry I just couldn't let that go.