Mac OS X is unique in that running out of HDD space will not dramatically slow your computer down like a Windows PC. It depends what you're doing on that Windows partition and which you use more often. For me, I only play games on Windows. Therefore, I set aside 300 GB for existing games and 150GB for future games. If you create more content on Windows, then you should increase your Windows partition size. The minimum for Windows should be around 64 GB for smooth running and a few programs.