Purchasing a purebred puppy has a major advantage in that you will know how large the puppy is going to get. You'll know the sizes of both the parents, and you can speak with the breeder about how large the breed usually gets. With mixed-breed puppies or puppies from a shelter, where the parentage is not always known, judging how large a puppy might get is a great challenge. There are several clues which will tell you how large the puppy will grow.
Look at the dog's family members. If at all possible, look at the dog's mother, father and siblings. Seeing the two parents together will let you see how large the puppy can get, because puppies will not grow to be significantly larger than their parents. If the dog is a mixed breed of unknown parentage, other puppies might have stronger features inherited from the mother or father, which can allow you to guess what the parents might have been.
Examine the puppy's skin. Puppies with very loose skin that looks baggy on them tend to be larger dogs.
Examine the puppy's paws. Paws that are delicate and seem more proportionate to the young puppy indicate a smaller dog, while puppies that will grow into larger dogs tend to have disproportionately large paws.
Weigh the puppy when it is 14 weeks old and double that amount. According to Pet Place, this can roughly equal the weight the dog will attain when it is fully grown.
Measure the puppy's height at the age of six months. The puppy's height as an adult will tend to be about 25 percent larger.