Small dog breeds are generally under 25lbs or 11kg and come primarily from the Toy and Terrier groups.
Not all small breeds are the same. Some are vermin hunters and some are companion lap dogs. Your lifestyle should reflect your choice of small dog as a Jack Russell Terrier couldn't be more different in temperament to a Pekingese if you tried.
Pros and Cons Of A Small Breed
Small dogs are often great for apartment living and are easier to manage.
Upkeep costs, vet bills, etc are usually much less than a big dog.
Small dogs generally live longer
Terriers can be very stubborn in training, Toy breeds are often difficult to house train.
Small dogs are more delicate and get hurt more easily