The most reliable way to estimate the adult size of a puppy is to see how big its parents are. Since height is determined by genetics, puppies are generally similar in size to their parents.
Mixed-breed puppies with parents of very different sizes may take after either one of them, so the adult size can be unpredictable. The puppy's paws may give a clue. Puppies that are going to be large adults tend to have large paws even when young, although this can vary by breed. Large amounts of loose skin may also be a sign that the puppy is going to be large, unless it is a breed known for loose skin, such as a shar-pei.
Most puppies reach approximately 75 percent of their adult height when they are six months old. Once a puppy reaches that age, it's possible to calculate the adult height. However, most dogs continue to gain weight until they are about 2 years old.
A less accurate way to estimate a young puppy's adult weight is to weigh it when it is 14 to 16 weeks old, then multiply that by two. The reliability of this method can vary widely and is particularly inaccurate with large breed dogs, such as Saint Bernards, because they tend to grow more slowly and put on most of their weight later.