There are many breeds of dogs that are bred in a 'toy' or 'teacup' size, most commonly Chihuahuas. These tiny creatures fit in the palm of your hand. Be aware though, they have a very short life span. Look at Miniatures instead for a companion that will have less complications.
Chihuahuas are the smallest breed of dog but beware of buying a dog just for size. Puppy mills and Backyard breeders often breed for size alone and the puppies can be born with health problems and birth defects which will cost you a lot of time, money and heart break.
Some examples of very small (toy) dogs are Chihuahua, Teacup Chihuahua, Miniature Yorkshire Terrier, Papilion, Shih Tzu, Pekingese, Pug, Boston Terrier, Lhasa Apso, Petit Chien Lion, Maltese and Bichon Frise.