Just for reference, here's the median size of my zipped ROMs (most emulators can play zipped):
NES: 128KiB
Gameboy/Gameboy Color: 279KiB
Genesis: 713KiB
SNES: 1088KiB
Gameboy Advance: 5216KiB
Pretty much, anything that doesn't come on a CD isn't going to be taking up a significant amount of space. CDs will be quite a bit bulkier, and I think they average about...50MiB? All of the programs for the Wiz probably won't take up more than 100MiB. Overall, I don't think anybody really needs more than 2-4GiB of space.