The average Lionhead rabbit will reach a full size between 8 and 10 months old. This is the age when they will have reached their full potential adult weight, which is between 3 and 4 pounds. However, it is important to note that their coat will continue to grow and thicken throughout their lifetime. As a result, Lionheads may appear larger than they actually are due to their thick fur. In general, though, a Lionhead rabbit is considered fully grown once they reach 8-10 months old.