It really depends upon what you are going to install to it. Normal Operating System plus a few programs - 120GB is fine. If you have a lot of programs to add to it, you may need 250GB.
I have Windows 8.1, Microsoft Office, Microsoft Visual Studio Professional, Adobe CS6 Master Collection + a few other programs, and use about 85GB of 120GB. You can utilize your 1TB HDD for the other programs....