How big is a full Windows XP install?
(Move this where ever appropriate)
Very.
Depends if you want it useable (like, with updates). With all updates and a few choice pieces of software, probably about 2gb, maybe close to 3.
A straight-up off of the CD install is about 1.5GB. After patching the required things (Blaster and a couple of the big exploits are the only things I'd patch) it's about 1.8GB ish.