Hi I recently purchased a Perfect Fit harness for my 5 month old puppy, Finn. I emailed for advice thinking I needed to order a 20mm L-S-L harness, based on his size/growth rate. You emailed back to say that I would need a 20mm M-S-L, so that is what I ordered. The harness arrived; it is great quality and comfy for my puppy, but it is almost at the full extent of adjustment already, so I will need to buy additional pieces very soon for it to fit correctly. I think I was right in thinking I needed a L-S-L in the first place. So I am a bit disappointed on advice given, as I will need to buy new larger parts quite soon.