Size：Optimal for a head circumference of 57-60 cm. The maximum head width is 18 cm. The maximum head length is 24 cm（fit for most adult）. If the cheekbones are wider than 14 centimeters, there is a risk that the details of the jaw mechanism will press on the face. if the head is longer than 24 cm, the mask will not be able to wear! Be careful when measuring your face.