Judging by the assets in the Alpha, the following might be an indicative list of areas:
Cyseal (Overland)
- Tunnel (where you meet Arhu's creation)
- Black Cove
Homestead (Shelter Plane)

Sankima Pass
Farmlands
Lakes
- Cave
Hollow Marshes
Luculla Forest
- Mines
Thorn Lair
- Cave I
- Cave II
- Tomb
- Cave
- Monk Grave I
- Monk Grave II
- Monk Grave III
The Prison Garden of Stone King
Aleroth (whether this is included as a Demo only or is part of the game)


So when Larian had previously estimated that Cyseal (including bits that are fenced off) is ~1/4 of the world map, that seems very plausible.