All the Windows environments for Macs require a lot of empty disk space, although the amount varies considerably depending on which software you're using, which version of Windows you have, how many Windows applications you intend to install, and other factors. Apple requires at least 10 GB free for Boot Camp; for virtualization software, 5 GB of free space is a bare minimum (and more is better).
Under Boot Camp, you're stuck with whatever size you make your Windows partition. But with most virtual machines, the disk images you create can grow dynamically to one extent or another. So be sure you have more disk space available than the size of the virtual disk you start with.