Small: 11" - 15" chest girth, Toy Poodle, Yorkie * Medium: 15" - 19" chest girth, Beagle, Jack Russell * Large: 19" - 23" chest girth, Pug, Boston Terrier * Extra Large: 23" - 27" chest girth, Cocker Spaniel, Medium Collie * Extra Extra Large: 27" - 31" chest girth, Boxer, Doberman, English Bulldog