Harnesses are sized using the GIRTH measurement 

Head/neck hole sizes are as follows for guidance if your dog has particularly hairy ears (spaniels etc) or is a bull or cross breed dog, along with suggested breed examples for each size of harness (girth being the consideration). Please ALWAYS measure your dog as the information given below is a guide only

head neck of Size 0 harness up to 30cm  example: small chihuahua, puppies, cat

head neck of Size 1 harness up to 31cm example: cats, large chihuahuas, larger puppies

head neck of Size 2 harness up to 38cm example: toy poodle, pug, sml terriers, Poms, working cocker

head neck of Size 3 harness up to 43cm example:  Beagle, CKCS, Shelties, Min Poodle, Cocker

head neck of Size 4 harness up to 50cm example: Springer, Collie, Belgian, whippet/Lurcher, SBT

head neck of Size 5 harness up to 54cm example: Std Poodle, Setters, large SBT, Retrievers, Doberman

head neck of Size 6 harness up to 60cm example: Mastiff, Weimeraner, GDS, Rottweiler

head neck of  Size 7 harness up to 70cm example: Newfoundland, Akita, BMD, Leonberger