George Lucas needed a tall actor who could fit the role of the hairy alien Chewbacca. He originally had in mind 6 feet 6 inches (1.98 m) bodybuilder David Prowse, but instead Prowse chose to play Darth Vader.
This led Lucas on a search which turned up Mayhew, who says that all he had to do to be cast in the role of Chewbacca was stand up.
His height is a product not of gigantism but of an overactive pituitary gland secondary to a genetic disease called Marfan Syndrome. His peak height was 7 feet 3 inches (2.21 m)