I didn't know that Morrowind was smaller than Oblivion.
Don't worry, you're not the only one who didn't realize that, if these forums are anything to go by.
Cyrodiil is the biggest province in Tamriel, so why guess? I'd say maybe 35 square kilometres is what they'll make of it, though nicer would be bigger (without loss of quality, of course).
The size of the provinces in lore doesn't really have much to do with it, as the games don't necessarily need to all use the same scale. I think we can safely assume that Oblivion would have been much bigger if it had been on the same scale as Morrowind.
Anyway, I'm guessing the size of the map will either be comparable to Oblivion, or somewhat bigger, a size comparable to Red Dead Redemption might be within reason, but I doubt we'll see a world as large as Just Cause 2, after all, I can't think of any other recent game I've played with such a large world map.
As far as what I want, I wouldn't mind a map larger than Oblivion, as long as it does not come at the sacrifice of the quality, detail and variety of the landscape. I like having a large world to explore, but if it comes at the expense of making the world feel boring, then I'd rather it be smaller.