So, it definitely seems like the gameworld will be like this:
You start out in a small, self-contained, open level/world -- acts as a tutorial for the game, introduces you to the mechanics, lore, quest style.
Then, eventually, you get introduced to the actual open world (Separated from the "tutorial" zone by a loading screen.) -- a continent with 2 "zones" (As in, separated by a loading screen.). The first zone, the mainland, is (Probably.) the main/largest part of the game, and is apparently bigger than the world of Skyrim. The second zone, the islands, is, presumably, also open world, and probably much smaller.
So, 3 zones, at least - tutorial, mainland, islands. I assume there'll also be sub-zones, like dungeons separated from the zones by loading screens. There might also be other open world, or linear, overworld zones, but they'll probably be a lot smaller than the 3 "main" zones
In other words, it's like Morrowind, Oblivion or Skyrim with the expansion packs, just structured differently (Starting zone is a small, open world tutorial "area".).