With all this data that gets recorded (including even players' clicks and viewscreen positions) and with the capability of rewinding to any position in the game, what's the average size of a replay file?

I ended up sharing a CD with someone else and got banned for something that person posted.

Yes, replay files are very small. A few hundred KB at most for a longer match. Since none of the artworks, models, maps etc. need to be stored in a replay it doesn't take a any more space than a StarCraft: Brood War replay needed.