New
#11
50 to 60GBs should be ample - I have 40GB and my Win7 system with 1726 program folders is around 15GBs since months (no shadowstorage). For a data partition that is different and depends completely on your planned files. With e.g. videos, you can fill 500GBs in a few months. But then you can always offload to an external disk.
PS: I have my system on a 80GB SSD which I split 40/40 for system and high usage data. Other data is on a HDD.