Thanks! I don't know how much I'll really need, but after thinking about it again, 20GB seems good enough for me and allow some extra space. Also, like you said, I can always resize it if I need more.
The tutorial I found was for another version of Vista and had different menu options than my version. I found others, but I'm not sure if I understand. Basically, I right click on my OS drive, click on Shrink Volume, and then it creates another partition that includes the space that I just "shrunk" from the OS drive?