R.I.P. Slouch style hat. Size is determined by hook size
Small G (4.25 mm) Hook 20 inches by 9 inches long Gauge 14 dc = 4 inches
Medium I (5.50mm) Hook 22 inches by 10 inches long Gauge 12 dc = 4 inches
Large K (6.50mm) Hook 24 inches by 11 inches long Gauge 10 dc = 4 inches
American Crochet Terms. Pictures of many of the rows.