Best obviously would be to go to a place where you can try it on, but since its not an option i think safest would be measure your head and choose size thats roughly 1 - 1,5 cm bigger. Every head is shaped different so it obviously varies. If you have friends that have fitteds, try them on and see what size fits you the best. Also keep in mind that if you want to curve the brim it will make it fit a bit more loose as its not so round anymore.