These hat sizes are relative as a child can have a head circumference(HC) of 22″ and an adult woman can have a head circumference(HC) of 20″. I wear a 23″, which is large for a woman. So measure.
I have also read that the crocheted hat should be at least an inch smaller than circumference as the hat will stretch to fit. That is up to the wearer.
Note: When I list as Medium to Large I mean 21.5″ to 23″
And, small to medium would be Pre-teens to Adult woman.
Child (3 – 10 years) HC: 19″(48.26cm) – 20 1/2″(52cm).
Pre-teens and Teens HC: 20 1/2″ – 22″(55.88cm).
Adult Woman HC: 21.5″(54.61cm) – 22.5″(57.15cm.
Adult Man HC: 23″(58.42cm) – 24″(60.96cm).