Each harness type has their own size charts. Below are a few examples of sizing.

7.7 lb Yorkie in a medium Soft brown	62.7 lb .Lab in a XXL Soft green	7 lb. Boston in a M Soft Jacket purple
16.5 lbs French Bulldog in large Vivien pink	13.6 lb Dachshund in medium Pacific	12.2 lb Schnauzer in a medium Flame
5.9 lb Pomeranian in a small Ritefit	5 lb Yorkie in a small Superior	11.9 lb. Pekinese in a large Diamond
16.7 lb Boston in a large Hunter	13.6 lbs Dachshund in a medium S&S	14.9 Bedlington in a large Ocean Mist
8.8 lb. Japanese Chin medium Mezzo	5.9 lb Maltese in a medium Spring Gala	15.4 lb. Westie in a large Baby Checkered
16.7 lb Boston in a large Halcyon	5.9 lb Maltese in a medium Classic	11.9 lb Pekinese in a large Prestige
11.9 lb Pekinese in a large Snowflake	8.8 lb. Japanese Chin in a medium Buttercup	13.6 lb. Dachshund in a medium Dotty
7.7 lb Dachshund in a medium Ritefit Combat Harness	5.9 lb Maltese in a medium Spring	6.2 lb. Boston Terrier in a medium Lattice
14.3 lb. Dachshund in a medium Uptown	9.9 lb. Poodle in a medium Argyle	9.9 lb. Mini Pinscher in a medium Gala
14.3 lb. Boston Terrier in a large Patrial	14.5 lb. Pug in a large Yuppy Puppy	12.1 lb. Pekinese in a large Love Leter