SMALL: 12"or Less
MEDIUM: 13" to 16" inches
LARGE: 17" to 20" inches
X-LARGE: 21" to 27" inches
XX-LARGE 28" or UP
GIANT: over 90 Lb

NOTE: These measurements are approximate and can vary slightly between individual dogs and different breed standards.
Chart Breakdown

SMALL
12"inches or less
Examples
Chihuahua • Yorkshire Terrier • Pomeranian • Maltese • Toy Poodle • Papillon • Russian Toy
* meaning that you can put it in the palm of your hand

MEDIUM
12" to 16" inches
Examples:
Bichon • Shi-tzu • Cavalier • Pug • Mini Schnauzer
* is a dog you can pick up, such as a Shih-Tzu

LARGE
17" to 20" inches
Examples:
Aussie • Border Collie • Schnauzer • Chow Chow
* is one that's too big to pick up from the floor

X-LARGE
21" to 27" inches
Examples:
German Shepherd • Retrievers • Doodles
* is one that too heavy like Germany, Australian doodle

XX-Large
Over 28" inches
Examples:
Bernese • Australian Doodle • Standard Poodle • St. Bernand
GIANT are any dog over 90 pounds.
* is one that little kids can almost ride like a pony