John Goodman's wry charm has something to do with the fact that his personality is so much smaller than his body. You'd expect a man who's this huge and in the past couple of years, he has gotten even rounder to have an outsize, aggressive disposition, rather like that of his TV costar Roseanne Barr. Instead, Goodman is miraculously genial. In King Ralph, he plays a Las Vegas lounge singer who becomes the king of England when the entire royal family is killed in a freak accident. (Though American, he's the closest blood relative they can find.) All of a sudden, he's in pig heaven. The movie is an eminently forgettable string of bull-in-a-china-shop gags: Ralph hits a homer with his cricket bat, he sits in the royal bath wearing his Imperial margarine crown and scarfing Milk Duds, he does a genuinely rousing rendition of ''Good Golly Miss Molly'' during a dignified ball. Like Kindergarten Cop, this is a high-concept comedy in which the gags seem to have been designed primarily for use in the movie's trailer. Comedy that is to say, show-stopping, rib-tickling comedy is based on surprise, and there isn't a single gag here that pushes the envelope. Goodman, though, is such an ingratiating performer that you coast through the movie right along with him.