I heard the area generatable is 4 time larger than the equivalent area on earth that is possible to geenerate.

He uses procedural Terrain generation, so there is not limit.
New parts / revisited will be generated on the fly.
I guess he only saves blocks/elements that where altered after the
terrain generation procedure, this limits the amount of
data that has to be saved.

To have the same block-elumination in gamestudio, you would need
to be able to alter each of the 6 block sides brightness
seperately.