There are many reasons for wanting a small dog breed. It could be because a small dog will fit better in a small apartment or just because small dogs are so cute.
It is important to remember that even a small breed of dog needs obedience training. Otherwise they can become yappy, bossy, bite, or cause a number of other problems. All dogs should have basic obedience training, whether you take them to obedience classes or just train them at home.

Toy dog breeds are, by definition, small breeds. Please refer to the toy dog article for further information.
Affenpinscher 7-8 pounds
Australian Silky Terrier 10 pounds
Bichon Frise 7-18 pounds
Biewer 5-7 pounds
Belgian Griffon 8-10 pounds
Boston Terrier 10-25 pounds
Brussels Griffon 8-10 pounds
Cavachon (cross between a Bichon Frise and a Cavalier King Charles Spaniel – see these breeds for sizes)
Cavalier King Charles Spaniel 13-18 pounds
Chihuahua up to 6 pounds; “Teacup Chihuahuas” may be especially small
Chinese Crested Dog 10-13 pounds
Cockapoo (cross between a cocker spaniel and a poodle) – toy: under 12 pounds; mini: 13-20 pounds; standard: 21 pounds or more

English Toy Spaniel about 11 pounds
English Toy Terrier less than 12 pounds
Havanese 7-13 pounds
Italian Greyhound 7-10 pounds
Japanese Chin 4-11 pounds
Maltese 3-10 pounds
Toy Manchester Terrier less than 12 pounds
Toy Mi-Ki 5-8 pounds
Miniature Pinscher 9-12 pounds
Miniature Dachshund less than 12 pounds
Papillon 3-9 pounds
Pekingese 7-14 pounds
Phalène 3-9 pounds
Pomeranian 3-7 pounds

Pug 14-18 pounds
Shih Tzu 9-16 pounds
Spoodle (another name for the Cockapoo – see above)
Tibetan Spaniel 9-15 pounds
Toy American Eskimo 6-10 pounds
Toy Fox Terrier 3-7 pounds
Toy Poodle less than 10 pounds
Yorkshire Terrier 5-7 pounds
Be sure to read the descriptions of these small dog breeds, since they can have very different temperaments. Not all small dogs are alike.