Most chihuahuas are considered full grown by the time they are one year old. The typical chihuahua puppy will be near its adult size when it reaches six months old.
No matter the breed, dogs tend to follow the same general rule with regard to growth. While final bone length and density, muscle growth and other signs of adulthood take longer in larger breeds, small dogs such as chihuahuas experience the same signs. Chihuahua puppies experience a growth in the first few months after birth; growth slows around the fifth or sixth month, and it is complete by 10 months to one year of age. Love To Know and other dog-related site points out that it is important for owners to note that various factors such as medical conditions, illness near birth or nutrition imbalances may affect the chihuahua's size, so the size of the dog should not be used to determine maturity.