AURA
Small:
Oval Head Shape: 51cm - 55cm | 20.1in - 21.7in
Round Head Shape: 51cm - 54cm | 20.1in - 21.3in
Medium:
Oval Head Shape: 55cm - 60.5cm | 21.7in - 23.8in
Round Head Shape: 54cm - 59cm | 21.3in - 23.2in
Large:
Oval Head Shape: 60.5cm - 63cm | 23.8in - 24.8in
Round Head Shape: 59cm - 61.5cm | 23.2in - 24.2in
When in doubt between Small & Medium or Medium & Large, always opt for Medium.