My understanding is that some vets (Banfield/Petsmart, for one) give a standard dose, but others adjust for size. One of the vets we use has a sign on the wall stating they believe in vaccines tailored to each dog. He reassured me that the doses given to Mickey and Piper were NOT the same as those given to a German Shepherd. Are there others vets in the area you could check with?
He prayeth best, who loveth best, all creatures great and small.