The Myo armband has been designed to fit an arm with a circumference between 19 cm and 34 cm (7.5-13.4 inches). The armband should fit comfortably snug, not sliding on your arm when you move. You can use the sizing clips to further tighten the fit to your personal preference. If you find the armband too right can try putting Myo around something that has a similar circumference as your forearm and leave it for an extended period of time or overnight. The material the armband is made out of does expand and contract based on usage.