If you need to put sample libraries on the C: drive, find out how much space they require and then add 128GB, which should leave room for any surprises. Otherwise, I would use a 128GB SSD and use a separate drive for sample libraries(i use a WD Black 2TB for my samples). Yes, you COULD use a 64GB SSD as a system drive(and I have before), but the price for a 256GB on sale isn't too hard to swallow. I am currently using a 500GB SSD, but the actual used space is more like 220GB, but I am no where near as diligent as many of the other users here when it comes to space efficiency(I'm too busy backing things up
)