How much hard drive space do I need to install Windows 7 on my Mac via Boot Camp?

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).

Windows 7 System Requirements

16 GB available hard disk space (32-bit) or 20 GB (64-bit)

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.