I use three 5" feathers, but been thinking about going to four 4" because "full length" feathers keep getting shorter and I am more likely to get two cuts per feather with 4"than I am with 5".

As far as flight, I haven't been able to tell a difference. With three 5" feathers, I get 15" of feather on the shaft. With four 4", I get 16" of feather. I have mine matched so I can shoot bare-shaft though, so feathers just help cover my mistakes and keep broadheads flying straight.

Chad