The jersey is 24" around total or 12" laying flat. 19" long on back side. The front/belly area is Vneck and is 13" fom bottom of V to hemline. The sleeves are 6" laying flat or 12" all around. I hope this helps. I am not too sure other than measuring. My dog is 35lbs and it depends solely on the maker of things I get for him. He out grew one comfort harness that was medium and then I got a new one by a different maker and it swims on him yet is a medium. So best to know measurements and go by that.