Rockstick Ergonomic Mouse RS1201M Black and Gray Small/Medium Determine size by measuring the width of palm at the knuckle point. 2.75" - 3.75" is Small/Med 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