I'm seeing comments recommending 20 and 30GB installs. I'm wondering if these guys have boot camp at all.
Initially, I tried the minimum install. As soon as I got up and running I was getting 'low disk space' warnings. Then I tried 30GB, which seemed like a good size, but once I installed one (rather large) program (autodesk revit) I was out of space again.
Finally I set it to 50GB which is enough for a few large programs and some space for files. I would't recommend going any lower.
I have bootcamp on a 13" 2011 MBA, Win 7 Pro SP 1 with all updates current. Only a couple of games installed, including Civ 3. No other huge programs. I set the Windows partition at 40gb. I now have 19.5gb free. So, yes, you can get by with a very minimal install at 20gb, although 30 is probably as low as I would go, at least for Win 7 Pro. I have no idea what Ultimate or any of the other Win variants would require. There are also slipstream Windows installs that are much smaller than the normal install. They leave off things like multiple languages, fonts, etc.