I used 30 gb because I figured I'd be using it more than I actually am.. To play a couple of games etc.. Turns out I hardly ever use it and I would have prefered to have used 25 or less in hindsight. A full harddrive slows down the computer a lot(talking about the more important OSX partition here).
If you're not planing on using the boot camp partition for games or don't need it for other reasons I would seriously consider getting Parallels/VMWare/Fusion. I used Parallels for streams before and it worked fine. You can't catch viruses or other malware using a those programs. That's what I've heard anyway.
