1 1/16 to 1 1/8 for fixed blades.Ive used smaller but wasnt impressed with the blood trails. You couldconsider an expandable also. Even with a perfectly tuned bow any flaws in your formwill show up dramaticaly morewhen you are shooting fixed blade broadheads.The newer style super short heads and expandablesare more forgiving to shoot because of the reduced blade surface but still carry good cutting diameter.