This pattern was written for an adult, but can easily be modified for larger or smaller head by using different multiples of 10.

Cast on 80 for 22” circumference
Cast on 70 for about 20” circumference (small headed woman)
Cast on 60 for 18” circumference (child)
Cast on 50 for a baby.

Use whatever needle is required to get gauge and then multiply that number by needed size. I found rounding down makes more sense as this stitch does have plenty of stretch.