Small child (under 6)
Large child (6-11)
Adult Small (suitable for average 12+ and smaller than average adult heads)
Adult Medium (average woman, smaller than average men)
Adult Large (average man, women with larger heads)
Adult Extra Large (larger heads, particularly good for men with full beards)
These are huge generalisations, of course, but if the masks are too big a couple of stitches in the elastic loops can shorten them, and if too small I can exchange and replace.