I would use gradual exposure.
- I would have the client inside and they can see dogs of different sizes outside the window.
- I would have the client sit down and bring in 1 small dog to be in the same room but not within 6 feet of the client.
- I would have the dog be within 6 feet of the client.
- I would have the client pet the dog.
- I would repeat these steps with dogs of varying sizes.