As has been told to me recently, the hat 'size' is the least accurate way to determine if a hat will fit you. Your head size, in inches or centimeters, is the final answer as to a properly sized hat. Different manufacturers' 7 3/8 (hat sizes are based on a formula), for instance, can be sized differently. That is why when buying a hat, a more knowledgeable hat maker will ask you for your head size in inches or centimeters, at the very least, not your hat size. Even better is a conformer to get your shape, as well.
So, when buying a hat, the best thing to ask for, if you can, is the measurement around the sweatband.