Tooie-sized worlds without the expansions, and the expansions adding sizable chunks to the levels.
We're talking a modern game here, so I'd expect to see very large levels. Definitely not N&B scale, but starting at Tooie's size and growing would provide plenty of room for interesting things to do in the level. The "main chunk" should be as densely packed and interesting as its expansions provide.
In particular, I feel that the expansions should not make it feel as if the "main chunk" of the level is small compared to the whole. Maybe more like 60% main chunk with Pagie expansions pushing the boundaries to 70%, 80%, 90%, 100%, 110%.... etc (expansions should feel like you are getting really deep into the level, with that ever so elusive sense of exploration).