Measure dog’s height to the shoulders, and length from chest to rear of leg, then add 4"-6" to each measurement. See below for estimations by breed. For approximation only.
Small: Chihuahua, Pekingese, Cats/Kittens, Rabbits/Ferrets.
Medium: Cats, Chihuahuas, Dachshunds, Pekingese, Yorkshire Terrier
Intermediate: Pugs, West Highland White Terriers, Bichons Frises, Lhasa Apsos
Large: French Bulldogs, Border Terriers, Miniature Schnauzers