You'll have to figure that out based on your storage needs. I have what I believe to be a lot of games, Crysis, Crysis2, Crysis Warhead, BF3, L4D2, COD Black Ops MW3, Fear3 Sniper:Ghost Warrior, Fuel, Need For Speed:Shift, Metro 2033, and many more, and I'm using 233GB of SSD space, if that helps any.
My primary SSD is a 240GB Mushkin. The OS, and all my software, benchmarking apps, etc. only total 96GB.