You could use a 64gb that is actually what i use at the moment, but prices have fallen recently you can now get a 120gb for less than i paid for my 64gb.
Depends on how much software you use and the size of it.
If you just use some basic free programs 64gb will easily be enough, but if you use a lot of bigger programs like, microsoft office, adobe photoshop etc, then a 120gb would be better.