Average:
First couple of hundred, 128Mbit (16 MByte) to 256Mbit (32Mbyte)
It then got higher and had some more in the 512 range.
Nowadays 512 is probably the usual, actually I just checked and 512Mbit games just took the lead in the last 4 releases.
Bear in mind roms get trimmed (sometimes quite a lot) and you can also rip junk (intro movies, sound, other languages...) quite simply from a rom: http://www.sosuke.com/ezflash/viewtopic.php?t=457