"his head and leg size will determine how good he is" that has to be the biggest load of shite I've ever heard, the size and growth of a dog has f**k all to do with how good the dog will be, I've got a Plummer who has really long legs and a small'ish head and I dig to this terrier every weekend, in shows it will benefit the dog to have average size head and legs depending on the breeding standards but when working the terrier it has nothing to do with size, this is proven by the fact that there's so many different sizes of terriers for example the in Germany daschund 'sausage dog' are used to