I wouldn’t go less than 128GB. At first it will seem like you have quite a bit of extra space, but as time goes on it gets eaten up quickly. Ideally I’d get a SSD that can fit all your programs, I find it’s a bit of a pain to have programs split over multiple drives.
I just upgraded to a 256GB and it is awesome, it fits all my games and programs with a little wiggle room for random files that haven’t been moved to my HDD yet
If you use your computer a lot and will be installing/uninstalling, downloading, etc go for the 256GB if you can afford it. If you stick with a pretty standard set of programs and they will all fit on a 128GB then get that.