I have a Macbook Air and would like to install Windows 7 on it via Boot Camp. How much hard drive space should I make sure to have free before doing this?
Minimum for just install is 20gb for x64 and 16gb for x86.
Beyond that the question is what programs do you intend to run.
Personally, I find that 60GB is decent if you're frugal with your programs (that's photoshop, a few games, and some project files for C++, etc. in my case).
Again, depending on your uses - you could probably get by on 30-40.
Here is an easy way to save space
Once installed, click "My Computer" and then Right Click on the white area and select "Properties". In the pop-up window, select "System Protection" on the left and disable the Windows Restore points. They are useless and gobble up space.
Windows 7 needs about 16–20 GiB (32 and 64 bit, repsectively) in a fresh install. Also remember to leave some space for applications.