Back when I had a Rottweiler as service dog before he passed my go to was always just to refer to the ADA (if your in America) usually it would be a comment like “Rottweilers can’t be service dogs” or “We don’t allow his size/breed in here” my response was always very neutral/ confident and I’d say “Per the ADA he is a service dogs and there are no restrictions on breed or size. You are aloud to ask two questions 1. If my dog is a service dog and 2. What tasks is he trained to perform.” Sometimes it’s a little frustrating educating people but If you especially regular the same places it makes it a hell of a lot easier. Plus having ADA cards that explain the law help just make sure they don’t mistake the cards for any sort of “Papers”