MEASURE YOUR HEAD
Size Small: de 50 a 53 cm
Size Medium: de 54 a 56 cm
Size Large: de 57 a 58 cm
Size Extralarge: de 58 a 60 cm
You can use a tape measure(or a thread that you will have to compare next to a meter) to determine the lenght.
Place the string or ribbon around your head above your ear.
Hold the tape firmly, but not too tightly.
Basically, you need to measure your head exactly where your hat is going to be. It is important that you feel comfortable.
If you are in doubt between two sizes, please choose the larger siza.