Adopting a mixed-breed puppy or young dog, whether from an animal shelter or rescue or by picking up a stray, can come with some variables, particularly if the youngster is of unknown parentage. Not knowing how large a mixed-breed dog will grow could become problematic for those in smaller homes or apartments, those without a yard or those with small children or other pets. While you can't predict with certainty the eventual characteristics of a mixed-breed dog, you can use what you know about the pup to make educated predictions.
Consult a veterinarian. If your mixed-breed puppy's ancestry is not known, a veterinarian should be able to recognize the main breeds in your dog's background and thereby give you an idea as to how large your dog will be in adulthood.
Research the breeds in your mixed-breed puppy's background if this information is known. There are national clubs for most purebred dog breeds; these clubs host informative websites and publish literature about their breeds and will discuss physical characteristics such as size ranges.
Check your dog's paw size. Larger paws portend greater height and, therefore, more potential weight. While this won't give a hard-and-fast representation of how large a dog will be, puppies with small paws shouldn't be expected to grow large, and vice versa.
Look at the puppy's parents, if possible. A mixed-breed puppy may come from mixed-breed parents, in which case looking up breed information will be of little help. Seeing how large the puppy's parents are will likely provide an estimation of the dog's future adult size.
Use the "double-it" formula. Check the puppy's weight at 14 weeks and double it to get an approximation of his future adult weight.