XS
Parameters: Back length for two wheels 22~33cm | Back length for four wheels 27~38cm | Chest circumference 36~46cm | Back height <31cm
Reference weight: 1.5-4kg
Suitable for: Small dogs ( Toy Poodle, Chihuahua, Pomeranian, Maltese, Yorkshire Terrier, small mixed breeds (puppies / adult small dogs) )

S
Parameters: Back length for two wheels 22~33cm | Back length for four wheels 27~38cm | Chest circumference 42~52cm | Back height <35cm
Reference weight: 4-9kg
Suitable for: Bichon Frise, Schnauzer, young/small Corgi, Pug, small terriers, Shih Tzu, Pekingese

M
Parameters: Back length for two wheels 35~48cm | Back length for four wheels 40~53cm | Chest circumference 46~56cm | Back height <36cm
Reference weight: 8-15kg
Suitable for: Standard Corgi, Shiba Inu, French Bulldog, Shetland Sheepdog, Cocker Spaniel, medium mixed breeds

L
Parameters: Back length for two wheels 35~48cm | Back length for four wheels 40~53cm | Chest circumference 50~62cm | Back height <40cm
Reference weight: 12-22kg
Suitable for: Border Collie (smaller build), Whippet, Dalmatian (smaller build), Staffordshire Bull Terrier, medium-sized domestic mixed breeds

Additional notes
Short and stocky dogs (English Bulldog, Pug): Increase weight by 2~4kg for the same size;
Slim breeds / Greyhound types: Decrease weight by 3~6kg for the same chest circumference;
For wheelchair selection, prioritize Back length and chest  circumference data, weight is only for reference.