To achieve a mask that fits you well, here is a reference chart for the various sizes.
Measure Widge of Face from ear to ear (just in front of the tragus. Make sure to measure across the bridge your nose. Length is measured from the bridge of your nose to the tip of your chin.
Size Face Width Face Length
Small Child 8.75 inches 4.375 inches
Child 9.5 inches 4.75 inches Small/Teen/Adult 10.25 inches 5.125 inches Medium/Teen/Adult 11 inches 5.5 inches Large 11.75 inches 6 inches X-Large 12.5 inches 6.375 inches