How much space for Bootcamp WIndows OS only
Microsoft's minimum requirements for Windows 8 are 16GB for 32-bit and 20GB for 64-bit (http://windows.microsoft.com/en-ca/windows-8/system-requirements).  You may not plan to install anything else, but likely will like Acrobat, VLC and other utilities you will deploy over time to support day to day use of the OS.  Also with the typical updates of the OS, etc. space in the disk will fill up.  That said I would recommend you double the minimum requirements to ensure your disk has the room you need over the long term.  Another option would be to run Windows virtualized in something like Parallels.  I understand the benefits of Boot Camp...but the benefits of virtualization trump the performance hit for me.