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From the screenshot it looks like there is going to be a huge lake which will cover most of the map. I would guess the "land" area will be about the same size as gta san andreas.
I don't know exactly how big the map will be, but I know you're definitely wrong on this. The lake you're talking about is the game equivalent of the Salton Sea, which is set in the desert region of the game. It may take up a large part of that part of the map, but from everything else we've seen and know about, that lake will be a fairly small puddle compared to the entire world.