Jensen Synthetic 40mm ISU Lg. RHS (134R) How to determine size: Bare arm or with T shirt only, measure the arm diameter bent 90 deg. at the elbow and in the upper biceps area. For all leather slings: 11.5 inches dia. = size small 12.5 inches dia. = size medium 13.5 inches dia. = size large For all synthetic slings: 10.5 inches dia. = size small 11.5 inches dia. = size medium 12.5 inches dia. = size large The slings are designed to account for the bulk of a heavy shooting jacket plus at least 1 sweat shirt underneath but DO NOT wear these items when measuring the arm.