The short answer is Golden Retrievers and Labrador Retrievers. There are exceptions of course. Working dogs are easy to train but tend to be protective. Field dogs tend to more interested in their environment than people. Small dogs can't pick up large objects or pull wheelchairs.
Large dogs are hard to put under a table in a restaurant or out of the way on a bus or plane.
A good Service Dog is not protective, is people oriented, is not overly active, and is confident but not dominate or submissive. Since most Hearing Dogs are rescued from shelters, most of them are mixed breeds. They come in all different sizes, shapes, and colors. The great majority of Hearing Dog applicants request small to medium sized dogs, so most Hearing Dogs are Sheltie size or smaller. In addition to size, personality and temperament are important for a Hearing Dog.