I'll concede that I can't find an official source for the scale. I found it on a map, but that isn't official I've realised. Nonetheless, the idea that we can't make a game out of High Rock is still absolutely stupid. There is no scale 'the rest' of the TES games are made at. Oblivion is scaled down more than Morrowind and Morrowind is scaled down WAY more than Daggerfall (Which is larger than England), yet Skyrim is scaled less than Oblivion (same size in the game, but smaller on the map). You can't say "the size the last game was is the size of the world.". If they made a game of Daggerfall, IT WOULD be the latest game, therefore, by your logic, it can be whatever size it wants to be. There is no constant, so you can't stop it because of that. Just because it's old doesn't mean Daggerfall isn't part of the TES series or anything. It still had a vastly different scale. There's no reason High Rock couldn't be big enough for it's own game.
Yes, there is scale. To say there isn't is asinine. If they remade daggerfall it'd be the same size as the map in Oblivion.
Cyrodil is proclaimed in the lore to be the largest province, if Cyrodil is the size it was in Oblivion, then Elder Scrolls: High Rock gets made and it's 200 km across, that'd be pretty inconsistent, woudln't it. In TES: Arena you can see the tamriel map, the provinces are obviously to scale. If Cyrodil in oblivion is the size that it is, then all other provinces are going to conform to that.
the maps are going to be getting smaller, not bigger. You can say "they can be whatever size" all you want, but it's not going to happen. If skyrim is 16 square KM, then for all intents and purposes, so is High Rock.
The tamriel map as displayed in Arena
http://images.wikia.com/elderscrolls/images/5/5d/Tamriel_map_arena.jpg