The "right" amount is a bit subjective. WinXP requires 1.5GB of space, but you'll also need room for additional hardware drivers, room for temporary space/virtual memory, and room for games. You'll need to consider how much space your games will need. On my 500GB HDD, I partitioned off 100GB. It's not nearly enough for all the games in my library, but it allows me to have several of them installed while running Win 7.
However, I don't think there's a right answer that anyone other than you can provide. Also, read the Bootcamp message and make sure it supports WinXP. I know the 2011 MBPs do not.