If big is what you want in your bike, then how a bout huge or mammoth? The Leonhardt Gunbus is the labor of love of Mr. Clemens F. Leonhardt. He has built "the world's biggest motorcycle", and that's not all, he plans on going into go into series production very soon. Powered by a "410-cubic-inch, air-cooled, pushrod V-Twin", the Gunbus is apparently fit for the road.

Source