~10-15gb for the Windows files, depending on how much you strip it down. Another ~5gb for swap file, so 20gb for your system to be safe. Then you will need whatever space the games would take up. I wouldn't go under 50gb, which doesn't leave you a lot for OSX.
Give 40GB a try. That's what I'm doing with my 128GB SSD. With applications like Steam you can have 1-2 games installed that you play frequently and leave the rest off.