Collar/lead size reference by dog size:
Small - 1/2" wide fits 7"-11" (Chihuahua, Maltese, Pink Papillon, Papillon, Toy Poodle, Miniature Dachshund, Yorkshire Terrier)
Medium - 3/4" wide fits 10"-16" (Beagle, Bichon Frise, Border Terrier, Cocker Spaniel, Cairn Terrier, Cavalier King Charles, Cavalier Blenheim, Dachshund, English Cocker Spaniel, Jack Russell Terrier, Miniature Poodle, Miniature Schnauzer, Norfolk/Norwich Terrier, Scottish Terrier, Shetland Sheepdog, Shih Tzu, Welsh Terrier, West Highland White Terrier)
Medium Plus - 3/4" wide fits 11"-19" (Australian Shepherd, Border Collie, English Springer Spaniel, French Bulldog, Pug, Petit Basset Griffon Vendeen, Welsh Corgi)
Large - 1" wide fits 12"-20"
X Large - 1" wide fits 15"-26" (Rottie, Bulldog, Irish Wolfhound, German Shepherd Dog, Golden Retriever, Labrador Retriever, Rhodesian)
XX Large - 1" wide fits 19"-34" (Bullmastiff)