I used 10GB for XP Pro. I have about 6GB left after installation and a few programs I use.
I only went with 10GB because my 160 GB drive is filling up fast. I only have about 40 GB of free space left on OS X. I'll probably need to upgrade to a larger drive in about a year, maybe less.
This is my first Mac, but it really hurt my feelings to install Windows on her.