Anywhere from 3" to 5" would work well.
If you're shooting shorter distances out to 40 feet (not yards), go for the biggest feather. After that, I suspect the larger sized feathers would start to slow your arrow down and the larger they are, the noisier they are.
Easiest way is to try 3, 4 and 5 on a few arrows and see what works best for you. :wink: