They are the largest penguins of the stiff-tailed family , which also makes them the largest penguin outside of the two giant penguins , the Emperor Penguin and the King Penguin . Adult Gentoos reach a height of 51 to 90 cm ( 20 â `` 36 in ) , making them the largest penguins outside of the two giant species , the Emperor Penguin and the King Penguin .