You should have projects and sample libraries on separate drives to this, so ANYTHING more than a 120GB is wasted for an OS drive.
Just make sure you:
a) Do not install any sample libraries on it, including the 10+GB that Cubase installs there by default.
b) Turn off Hibernation, AND then delete the file.
c) Have plenty of RAM for your needs, and set virtual memory to just a couple of GB.
I have a dual-boot Win 8.1 system, and both 120GB OS drives have enough spare room to keep a spare partition of 30GB+ (good for Windows, program or driver installation files, just in case).