10GB should be fine. That gives you room for a) the OS, b) the swap file (about 1.5 times installed RAM), and c) a Program Files folder for your Windows programs. That should do pretty well.
Minimum size is at least 2GB and probably more like 4GB-that's not just what the documentation calls for, but the practical requirements to have room to do anything. If you were working on a machine with multiple hard drives, you could tweak the XP installation so that JUST the OS was on the first partition, and with that you could probably get by with 2-3GB. But with your Mac, go with 10.