Measure your dog's neck where the collar will sit. Extra small; Bichon, Chihuahua, Pekingese, Beagle, Dachshund. Extra Small adjusts from 6 to 12 inches. Small; Poodle, Westie, Lhasa Apso, Poodle. Small adjusts from 9 to 15 inches. Medium; Airedale, Cocker Spaniel, Corgie, Dalmatian, Pug, Springer Spaniel, Basset, Boxer, Bulldog. Medium adjusts from 12 to 18 inches. Large; Collie, Doberman, German Shepard, Standard Poodle, Labrador, Weimaraner. The Large adjusts from 15 to 21 inches. Extra Large; Golden Retriever, Rottweiler, German Shepard. The Extra Large adjusts from 18 to 24 inches.