In general, children up to 7 years old use Small, and older kids use Medium. Average sized women use Medium, and average sized men use Large. Women and men with larger faces may need Large and X-Large, respectively.
You can determine the size you need either by FACE MEASUREMENT or by TEMPLATE. Unless you have a large or small head, most people could wear either of two sizes closest to their fit, by adjusting the ear loops and nose piece.
A proper fit is critical to achieve a tight seal guaranteeing the effectiveness of your A-Mask.