Measure nose to base of tail while they're lying down. That's the number to size from.
Up to 4 kg
Chihuahua · Pomeranian · Toy Poodle · Yorkshire Terrier
4–7 kg
Maltese · Papillon · Jack Russell · Mini Dachshund
7–11 kg
French Bulldog · Pug · Shih Tzu · Dachshund
11–18 kg
Beagle · Cocker Spaniel · Schnauzer · Whippet
18–27 kg
Border Collie · Boxer · Australian Shepherd · Springer
27–38 kgMost picked
Labrador · Golden Retriever · Doberman · Rottweiler
38 kg and up
German Shepherd · Bernese · Great Dane · Saint Bernard
Breeds are a guide — go by weight.
Between two sizes? Dogs who stretch out do better with the larger one. Dogs who curl up tight do better snug — the wall needs to stay close enough to lean into.