Chihuahuas generally finish growing between 9 and 12 months old. Like many small dogs, Chihuahuas grow quickly in their first 12 weeks of life, after which time their growth gradually slows.
A Chihuahua puppy weighs approximately half its adult weight at 16-17 weeks of age. This is a good time to weigh a puppy and get an estimate of its adult size. Chihuahua Club of America standards require a dog to weigh no more than 6 pounds when fully grown in order to participate in shows. Despite the show standard, a number of specimens of the breed are larger than 6 pounds and therefore better suited as pets for homes.