Don't forget also that additional space is required on a continuous basis as you install applications and Windows updates, plus sufficient free space for Windows to run properly, so your system partition size must be large enough to allow for that.
If you create a partition that's just large enough to accommodate a fresh Windows installation, that partition isn't going to be anywhere near large enough in a few weeks time.
I've had my Windows 8 on my 32GB SSD for a while and it is almost completely full. Only a few GB remain. If you are planning on getting an SSD, don't go for less than 60GB. It is very inconvenient to have to constantly clean the desktop, plus disable system restore and the paging file and stuff.