You can also stretch a hat that is too tight. In my experience, this is usually a better option than trying to shrink a large hat since the shrunken hat tends to stretch back out. Dampen the lower edge of the hat with warm water, and stretch it over an overturned bowl or saucepan which is the right size. Cover the bowl or pan with plastic wrap first to make removal easier and to prevent the transfer of any stains to the hat.
To re-size a MLB hat, place a cloth in a bowl of water, which you will use to moisten the edges of your hat. Then, turn the hair dryer on high heat to blow the center of your hat and this will make it to shrink a few inches. Check http://www.ehow.com/how_8744039_resize-baseball-hat.html for more information.