Technically, you can properly mix or master in any room size. But, I believe a smaller room is better than a very large one for someone who's just starting out. And when I say smaller I mean closer to 12'x15' than to 20'x30'. I've mixed and mastered songs for a number of years in a 20'x30' room. It took me a few days to get used to it, but after that I could do it.
The obvious problem with a big room is it's a very open space. If you don't have a good acoustic setup, the room will add reverb to every song. You have to compensate for this on every song you mix or master, because the extra reverb you hear isn't really in the music. It's coming from the room.
In a smaller room, even with no acoustic treatment, your mixes and masters will all sound more true. They won't be discolored from bouncing around a big room.