A typical Windows installation will require at least 16 GB, Parallels uses expanding disks, meaning the file that is used for the virtual disk expands on-demand, you just set the maximum size (32 GB by default) but it will only grow to that size if Windows uses all that space.
After installing Windows you will have already consumed around 8 GB. And then you have the page file in Windows which will occupy some GBs. Less than 16 GB won't get you nowhere.
I recommend turning off any snapshot features in Parallels, and hibernation in Windows, that will spare you some GBs. You might consider getting and external drive and offload some stuff to it.
Last edited: Jan 19, 2015