I'm not sure what the size is...the scenarios (there are two so far that I have had problems with) - the Imperialism scenario that is available here, and also "Years of Rice and Salt" available via the C3C disc.
I'm just hoping that whatever scenario modifications I've downloaded into Civ from users hasn't affected scenario loading. I don't know why it would, as it hasn't affected saved games or the Conquest scenarios, but all the same. No juice to the user-created scenarios...
I know I have plenty of room on my drive and plenty of memory also...
Its a conundrum.