The growth patterns and average adult sizes vary for each breed size group. Toy breeds typically develop faster than larger breeds and stop growing between 8 and 9 months of age. Small breeds usually stop growing between 8 and 12 months. Medium breeds reach their adult size between 9 and 12 months. Large breeds may take 12-18 months to reach their full size, and giant breeds can take up to 2 to 3 years.