I was looking through the West Marine catalog today and it explained how to chose the right fender. If my memory serves me correctly it said for the cylindrical shaped fenders they recommend 1" dia fender for every 5 ft. of boat length and 2" dia for every 5 ft. of boat length for the sperical shaped ones. Hope this helps.
http://www.westmarine.com/webapp/wcs...productId=6055