Calvin the Giant is a minor recurring alter ego of Calvin's. He appeared in a single Sunday strip, a single story arc and one piece of promotional material.
Calvin the Giant first appeared in a Sunday strip where he consumed some magic elixir (spelled "magic elixer" in the strip) that made him grow, in increments, until he was over 300 feet tall. The now giant Calvin then went on a rampage throughout his hometown, presumably his original intent when he made the elixir.
Calvin the Giant also made a disputably canonical appearance on the back cover artwork of The Essential Calvin and Hobbes. In it, he attacked a town much in the same way as in his first appearance, although he was not quite as tall. The town in the artwork featured the Chagrin Falls Popcorn Shop building: as such Calvin may have been attacking Bill Watterson's hometown of Chagrin Falls, Ohio.