Least possible size for a Boot Camp partition?

I have an rMBP with 500GB. I wanted to create a Boot Camp partition for 64-bit Windows 7 Home Premium, but making the partition as small as possible to save space. I'll be using Windows 7 SOLELY for games on Steam, all of which will be downloaded to an external HD. The only thing on the partition would be Windows 7 itself. I'm sure there are some files and programs built-in to the Windows OS that aren't necessary for gaming alone; can I delete pre-loaded programs or OS components to save space? I was hoping to shoot for a partition around 30GB; the only thing I'm worried about are the OS updates.

Many thanks.