Whatever you're comfortable with.........
Here-here! I think it also depends on the length of your leg bones. I only have little legs (they only just reach the ground when walking) and always chose 6 1/2" for comfort. People with longer leg bones would probably be happier on longer cranks.
The Victorian sprinter Zimmerman could do 40 m.p.h. on 66" fixed, which represents nearly 200 full revs per minute. That's twiddling! - mind you, he was on 6" cranks.
John.