Toy breed dogs (like Chihuahuas and Yorkshire Terriers) generally reach their full size between 10 months and 1 year. Small to medium sized dogs (like Beagles and Basenjis) will reach their final height between 12 and 15 months, and their final weight by 18 months. Medium and large breed dogs (like Collies and Golden Retrievers) are full grown by around 18 months, and reach their final weight at around 2 years. Giant breed dogs (like Great Danes and Irish Wolfhounds) may grow into their bodies by 18 months old, but it could take up to 3 years for them to reach their final weight.