The largest melodica keys I know of, Eric, are on vintage Hohner 36 key Melodicas, the Professional 36 (hard to find and usually very expensive) and the Piano 36, which is plentiful. If you look through recent posts by Lowboy Bootay, you’ll see that he has two of the Piano 36 for sale. He will sell at a reasonable price, and you can believe what he says about their condition. The keys are about the same in width but longer, about 4″ total, 3.75″ showing. It’s a noticeable difference.