I believe the best size would be small enough to be maneuverable and just large enough to launch a 9 to 12 lb. stone. This closely assimilates with the average, unfortunate, enemies thinking part. That is one of the things they tossed back and forth to terrify each other. Bowling balls are similarly made in the 8-16 lb. range. That game got started same way, if you don't have a fancy sling shot, grab it by the hair and sling it in there. Have fun.