The soft cloth portion of the mask is only available in one size and the headgear is available in regular and large. The most accurate measurement is directly under the jawline. If you measure 16 inches (41cm) or above, then you should consider the large size headgear. If you measure 16.5 inches (42 cm) or less, then you should consider the regular size headgear. The SleepWeaver 3D is intended to be used on individuals who are greater than 66 lbs (30 kg).