Measure your head where you want the hat to sit; generally, above your ears and eyebrows. If you don't have a measuring tape handy, you can use a string, then measure the string on a ruler.
If your measurement falls between two sizes, choose the larger size. You can adjust it with a size reducing tape.
"One size fits most" women's hats are generally 57 cm, medium. Men's one size hats are 59 cm, large. We will often note if a style "runs larger" and up to what size it will fit.
Sizes can vary some by brands. Fit and feel can change based on the material of the hat (soft, stiff structure), inner band type (elasticized, cotton, leather), style of the hat, or how you wear your hat (if for example you like to wear your hat tilted back measure your head on that path).