bearded dragon getting bigger

Question
My bearded dragon is 3 years old and 18 inches long he has a medium sized head and i want to know any foods to get him larger because he seemed to stop growing?

Answer
 In the US, an 18" male is considered a good adult size especially if he is a high color morph.  At three years old, he has been a full adult for a year.  A larger dragon would have to have the genetics to be larger, as from what is called a German Giant.