I already had one of these harnesses which I bought from the RSPCA shop when bringing our border collie home. He fit the medium size but it's on the biggest fitting so would not be possible to get a coat on underneath. Having been very pleased with the harness, I decided to get one for our stocky little Jack Russell cross dog who does get cold in winter and does like to wear a warm coat. I therefore read a lot of the other reviews before buying, with regard to size. After noting other customer comments re tight chest fit of the small size, I decided to try the Border Collie's medium size harness on him. On the smallest fitting it gives him enough room to feel comfortable and will give him enough expansion space to be able to wear his winter coat underneath too when needed. So now both the Border Collie who is himself quite stocky and the much smaller Jack Russell cross are both happy in the medium size! I'm glad I read the reviews first though! The harnesses are really great - well made, well finished to ensure they are comfortable to wear and I LOVE that it can clip into the seat belt fastening in the car. An extra feature in the harness allows you, if you prefer, to feed the seat belt itself through a built in loop on the harness. All in all LOVELY harnesses and well worth the cost.