Vvardenfell was a convenient cutoff point. Skyrim has no such thing.
This is quite true, and the last time Bethesda did a province that had no convenient cutoff point, they did the whole thing (Cyrodiil in Oblivion.) that is. While we don't know for certain that the game will cover all of Skyrim, it would seem likely. I think the really important question is not how much of Skyrim the game will cover, but how much that will be scaled down, because it seems unlikely that we'll get a game on Daggerfall's scale again. I think it's safe to say that, like Oblivion and Morrowind before it, Skyrim will present a version of the eponymous province that's smaller than what it should be in lore, but the question is, how much so? It may be that we'll see a larger map than Oblivion, this time around, only time will tell.
Tell that to the residents of Sutch.
That's different, though, because the area where Sutch was formerly located seems to still be in the game, so it still covers all of Cyrodiil, it's just that not all cities that were mentioned in the lore before the game are still present in its version of Cyrodiil, and Sutch got replaced with a fort of the same name. Just because all the landmass of a particular province is in the game doesn't necessarily mean every known city or landmark will also be in it.
Arena: All areas of Tamriel
Daggerfall: Highrock and Hammerfell
Morrowind: Just Vvardenfel
Oblivion: All of Cyrodil
Skyrim: ???
If you take Arena to mean Tamriel, as it seems Bethesda has justified the title of Arena by doing, then it could be argued that Arena actually covers all the areas its name would imply, of course, back when it was originally released, people who saw the game wouldn't know this.
And Morrowind didn't cover everything its name might imply, it actually covered LESS, by this same logic, Skyrim could potentially end up only including a part of the province too.