I ordered a PerfectFit harness for my 20 month old boxer girl who is a bit of a puller. I ordered using the online size guide which was 40mm L-M-L. On arrival I found the front piece a big loose even at the smallest so called up customer service. They called me back within a couple of hours & agreed to send a smaller front piece to try at no extra charge. This fit perfectly so I returned the medium one. First class customer service. The harness is great & my girl took to it straight away as it’s comfy & non restrictive. Highly recommend PerfectFit as a company for their amazing service & the harness as it’s not a “one size fits all” .