The Sonora is one of the many hats from the Geroge Strait collection by Resistol. It's a nice looking 4x 100% wool hat for any day of the week. It features simple black leather hatband, 3-peice hatband buckle, and leather sweatband.
The best way is using a tailor's tape measure and go by centimeters. Measure from around the center of your forehead. You can click the ruler size chart button on the product page and will show you the what size you should get.