Fitted hats have a size that can only be slightly shrunk, assuming their material is Woolite. Non-fitted hats have a plastic clip that facilitates a wide range of sizes. Determine your head size as small, medium or large and buy a fitted hat in that size. If you fall in love with a hat and it fits your head slightly looser than you prefer, you can shrink the hat using water and heat from the sun. Polyester hats cannot be shrunk, though most fitted hats from manufacturers such as 59/50, NewEra and FlexFit are Woolite.
Dip a sponge into water until it's wet but not fully saturated.
Rub the sponge gently along the inside rim of the hat.
Wear the hat under direct sunlight until the inside rim dries, shrinking the hat. It's optional to wear the hat, but wearing it as it shrinks may result in a better fit.