Penguins were once five feet tall.

The "water king" lived 36 million years ago and stood nearly as tall as a human! In relation, the tallest species of modern penguin, the emperor penguin, is just under four feet tall. The water king also had gray and reddish plumage, unlike its colorless contemporary cousins. The shift to monochrome plumes may have been the result of an adaptation to better hide from predators.

The whole article in National Geographic about the giant penguin can be found here.