You need to decide what, apart from the OS, you want on that drive. To be honest, 32 GB is pretty much just enough for the OS without programs. 64 GB is needed for programs too and for some, 90 or 120 GB would be needed. Remember you need to keep a certain amount of the drive empty for garbage collection and such.
For reference, I'm using ~52 GiB with a 4 GiB page file and 11 GiB of games (some more are on an HDD).