I was so happy with the one I bought for my lab that I bought another one for my other dog!
I have a 65lb male and a 55lb female and I am using the large size for both. I did order a medium for my 55lb female and returned it as it was way too small. Excellent quality and very easy to put on them. Also, adjusting the correct size on the harness was simple as well. Highly recommend.
I am so pleased with these harnesses. They were easy to adjust to fit and I love that you can get it on the dog without requiring them to step into it. I especially like the secure lock feature on the big buckle. We've just started using the harnesses, so I don't know about durability yet, but they seem high quality.