If you think one of these medium dog breeds might be a good fit for your family, read on to figure out which one suits your lifestyle best. Not too big; not too small: These dogs are the definition of a happy medium, and probably why they show up on our list of most popular dogs year after year. Their playful and kind personalities make for excellent additions to families. The way people define medium-size dogs can vary. Some organizations break down dog sizes into detailed groups, including extra-small, small, small-medium, medium, medium-large, large, extra-large, and giant. But, in general, small dog breeds tend to go up to about 20 pounds, and large dog breeds begin around 60 pounds. So anything in the middle would be a medium-sized dog. Because of this wide weight range—20 to 60 pounds—medium-size dogs make up a large portion of the breeds in existence.