Calvin the Giant appeared in the 11/13 through 12/2 1989 story arc. The origin of his condition was mystifying, as was the inversion of gravity that had occurred to him earlier the same day. Much like in his first appearance, Calvin the Giant's growth was continuous but also, uniquely to this story arc, infinite; in fact, he ended up outgrowing the Earth, and then the whole galaxy. However, he was instantly returned to normal size and whisked back to his room by phenomena bizarre as those that meddled with his size and gravity.
Bill Watterson said in the tenth anniversary book that he was going to have this go on for a month, but he decided not too.