We would suggest picking the size that has more adjustment room for your dog. For example if they would be on the largest adjustment of a size small and could happily go into the medium, It would be best to go for the bigger size. However we do have an easy exchange process, if the sizing is incorrect. We also have an online chat service, so if you are ever unsure you can give your dogs measurements to our lovely Nat on customer service, who will be able to help suggest the correct size.