Ergonomic Mouse RS1201L Black and Gray Large by Rockstick Determine size by measuring the width of palm at the knuckle point. 3.75" - 4.25" is Large. Determine size by measuring the width of palm at the knuckle point. 3.75" - 4.25" is Large. World's first vertical ergonomic mouse that allows users to click by utilizing the strength of the entire hand instead of the fingers alone Integrated palm support eliminates wrist anchoring Upright natural grip allows comfortable handshake position Memory foarm wrist-rest included for added comfort