Below is the recommended sizing for these fitted golf hats as outlined by Titleist. All measurements are in inches.
Small/Medium - Fits head size 21 7/8" or hat size 7"
Large/X-Large - Fits head size 23" or hat size 7 3/8"
Front Brim Depth - 3.5"
Back Brim Depth - 3"
To determine your hat size, measure the circumference around your head, keeping the tape level and firm, across the temples and above the eyebrow ridges. Once you've got your measurement, check the size chart to see what size is best for you. For in-between measurements, use the next size up.