It's really up to you. Do you think you'll be running a lot of programs on it? I think the default is that it gives WinXP 5GB for the install, but you can set it to what you think your needs will be as far as programs and files go.
On my 60GB I've set aside 15GB for XP, as I've also installed a bunch of games on it and want to have space for saving files (you can't use the rest of the drive while in XP, it only sees whatever space you set aside).
I gave it 20 gigs and formatted the WXP partition FAT so I can dl stuff to Windows-use when in OS X.
Later on I realized this was unnecessary as I have two external HD's formatted in FAT and I could have just used them. But 20 gigs should be plenty for now.