http://www.ancol.co.uk/guides/size-guides

.The Comfort Mesh Harness is made from lightweight, fully breathable air mesh material and strong nylon straps ensuring a comfortable fit for the dog whilst the owner maintains full control. Easy to fit, the harness simply slips over the head and has a fully adjustable chest belt to allow for dogs with a larger rib cage. The harness is secured using an Ancol quick release buckle.