I bought this harness for my 12 pound, 6 month schnoodle, took the measurements and bought a size medium. It is a tiny big too big but I pull the Velcro pieces together as far as I can to get a snug fit. I expect it will still work for him even when he’s full grown. It is easy to put on and my dog doesn’t even notice it while we are out walking. The other harness I had purchased for him constantly irritated him and he would writhe around until I took it off him. The harness helps to stop him from pulling and I can control the lunging he sometimes does when a bike or jogger goes by.