For the trend setting Doogie :) Protect your dog's eyes and head from the brightness & heat of the sun. All Doggie Baseball Caps have an adjustable toggle to secure the cap without over tighting so it is comfortable to wear for them. SIZE Chart SIZE Diameter Types of Dogs S 15 Chihuahua, Maltese, Schnauzer, Papillon stc. M 17 Boston Terrier, King Charles, Jack Russell etc. L 19 Cocker Spaniel, Border Collie, Basset Hound etc. XL 21 German Shppard, Labrador, Great Dane etc.