To find out the size of your head
Wrap a soft tape measure around your head, right over your eyebrows and ears.
A helmet should be fitted all the way around without gaps between the padding and the head, not too tight and not too loose.
Small: 52-55cm (20.5"-21.7") Medium: 21.7-22.8 inches, 55-58 cm large: 58-61 cm (22.8 - 24 inches)
A long-haired person may need to adjust the fit of the helmet.