I put XP Pro on my MacBook Pro using VMware Fusion. I gave it a 10GB allowance. Big enough to have a little breathing room for most anything I would decide to put on, small enough not to be too much of a burden on my 80GB HD.

The cool thing about VMware, though, is that while you tell it the max size, it will not take up that entire size right away. The Virtual Machine file for this install is not quite 4GB. That's all the space it takes on the disk for now, though it's allowed to ultimately take up 10GB if it gets that big. Seems pretty nifty, so unfortunate that whatever the setup is won't run in VMware or Parallels. (I can't really imagine why; if he's getting a MacBook Air, there aren't too many ways it can connect to things hardware-wise! I would think the camera info would come in via the network, and that should all work fine on VMware.)

Anyway, given that size and that I have installed rather little, even allowing for the fact that some of that might be VMware's overhead, I would make the partition at least 5GB to be safe.