If you are in a wheelchair you should qualify for a service dog.
These are NOT dogs you go out and buy and raise and train yourself. They are highly trained dogs that go through a process and then are "assigned" to their humans.
Size of dog depends on type of service you need. Some folks need bigger dogs to help them up and provide support, do various things around the house.
All of the above being said, a companion dog, an emotional support dog, does not need to be large or provide any service other than companionship and can be a service dog.
Not all small dogs are yappy or overprotective. They CAN be a little harder to housetrain because being smaller it's harder for them to understand that the ENTIRE house is the den and therefore a "no pee zone".
I'm not sure the type of dog is as important as a temperament that is compatible with your situation. An adult dog that is already housetrained with some training would probably be better for you as you can skip the very very intense puppy training period (it takes MONTHS to properly housetrain a dog) and with an adult dog you will be able to see it's temperament.
I think you should consider rescue dogs rather than breed dogs. They seem to have less health issues from inbreeding and will be considerably less expensive. Dogs that are available from rescue groups are often in foster homes and the foster parents have a great deal of information about the dogs temperament, training and personality.
I currently have two rescue dogs that have wonderful, open personalities and are bright and friendly to everyone, easy to train and easy to live with. They were both thrown away as trash (one dropped at a shelter with a horribly damaged leg, the other left newborn, umbilical cord still attached in a gutter in a ghetto) got into rescue organizations and I adopted them from there.
I suggest you go on Petfinder and Adoptapet, they are websites of adoptable animals. I also suggest you do some reading on training, specifically look at POSITIVE OR MOTIVATIONAL TRAINING vs TRADITIONAL OR DOMINANCE TRAINING. I used to use traditional as it was all that was out there but with my current two dogs use only positive and motivational training and love the relationships I have built with my dogs.
Another thing to think about when picking a dog is it's grooming needs. I picked dogs with short black coats that require little care. You can see my dogs on facebook by searching Sweet Georgia Pie and Kazoo Dog.
Good luck finding your new friend!