XSmall- 6mm Rope- Toy to Small Breed ... Yorkie, Chiwawa etc
Small- 8mm Rope- Small to Medium Breed .... Jack Russel to spaniel etc
Medium- 10mm Rope- Medium Breed - Large Breed.....spaniel to lab
Large- 10mm Rope /25mm Handle - Xlarge Breed
Every dog is different so It is up to you what thickenss of lead your dog should have, these sizings are just here as a guide.