I have seen a pomeranian that was a therapy dog, and dogs that are a little larger, such as spaniels. A Yorkie would need to be calm enough to pass the tests, and if too small, I wouldn't recommend that they become a therapy dog. Or you could limit where the dog went. For example, a geriatric psychiatric unit might not be a good idea, or even rehab with brain injured patients, who might unintentionally injure a small dog. Going the the library children's programs might work well though. Generally pet therapy dogs go to nursing homes, rehab units, and gero-psych units. Sometimes medical units too. It is suggested that they not go to surgery units, as therapy dogs have been cultured with MRSA on their coats. There are also specific guidelines of bathing, teeth brushing, etc. that needs to be done before the dog goes visiting.
