it's really hard to tell. head to toe (to me) would be how tall he is. head to tail would be how long he is, but is he large boned? What is his breed? Some cats are much larger than others.
Does he look too thin, or too heavy, is that why you are asking the question? What does the Vet say? Whenever you adopt a new cat, it's best to have them looked over by a Vet and get their kitten shots, get neutered, etc. anyway, so they should be able to answer that question more accurately.