Small Windows 7 on Bootcamp
Straight from Microsoft editions of Windows are all the same size, the feature sets are unlocked by what edition you've paid for. 


professional
enterprise
home and business
student 
ultimate  


All of them take up the same amount of space, but some have limited access to features. 



I would install the windows partition to an external drive.
You might be able to squeak by with 30GB if you have less than 4GB of windows files.