I would not go completely by size. Diamonds are finicky as it is, to keep your adults healthy it will do them best to wait until they are 4 years old. Why rush it. Let her mature before placing her body under strain from being gravid and having to lay eggs. Making her eggs would take a lot of calcium from her - diamonds in particular seem to be prone calcium deficiency more than other species. Do be in a hurry or you could risk her life and lose a gravid snake.