Puppies grow at vastly different rates, depending on their breed. Around 16 weeks, or 4 months of age, most puppies are considered half-grown, but this figure depends entirely on your dog's estimated rate of maturity based on his breed.
Breed Maturity Rates
Small dog breeds that mature at or under 25 pounds, such as shi tzus, generally are close to their mature height and weight at 4 months of age. By that measure, small and toy breeds are close to half-grown at 2 to 3 months. Larger breeds over 55 pounds, such as Labrador retrievers, reach full size at 12 months, so are about half-grown at 6 months. Giant breeds, such as Great Danes, usually reach maturity around 18 months to 2 years of age. These breeds are considered half-grown at 9 months to 1 year.