It's clearly more than twice the size of the island area in DOS2. I mean, you have to add the underdark map as well, the hidden cache of the thief guild, and the interiors like the temple, which is much bigger than the main priests house in the first area of DOS2.
Larian has stated the act 2 is the biggest one, so even bigger than what we've been playing so far.
As for the release day, my guess is 2022. Cause they haven't even finished to code the final act with its assets and cutscenes.
This is a huge game guys.
I think Larian missed a real opportunity to create the illusion of scope by not utilizing the overland map more. Right now everything is so crammed together it really forces you to suspend your disbelief.
If they used the overland map (like previous BG games and other cRPG) then they could have expanded the game more. It could have allowed more uses of history, nature and survival skills for instance by awarding successful rolls with small rewards or additional lore information. At the very least, it would have made the world feel bigger.