What is it?

How many Megabytes, Gigabytes, Terabytes?

I think I read somewhere that it is 4 Gig.

I think it depends on your computer.

But in any case, CS sets don't take up that much space, I know people with 1500+ sets and they're still fine.
I don't think we're ever, as a community, going to reach an amount of CS no computer can handle anymore.

I don't know exact stats though.

My thinking is along the lines of what Silvarret wrote just there. No limit. Depends on your system and how long you want to wait for it and the game engine to load all those resources.

It was only in RCT2 where there was a limit on the amount of .dat files one could have for scenery – I think that limit was just over 5,000.