It's as big a you fill it up...what's the question?
You misunderstood me. My question was "how to get the size", not "what's the size".
There is sp_spaceused, but this function doesn't work with temptables. The thing is, the user can create as many temptables as he wants. But this uses up the memory. So I need a way, to estimate the size of each temptable and the remaining memory.