Different types of CPAP masks require different measurements, which should be made with a ruler or tape measure while holding a neutral expression. Check with the mask manufacturer for the specific measurements needed to identify the correct mask size. Some manufacturers offer mask templates that, when printed to scale, show you the actual shape and size of the mask you plan to purchase.  If you find it difficult to select the right mask size, a sleep specialist or the staff at a brick-and-mortar CPAP retailer can check that your measurements are accurate and help you decide.