While the U.S. are one of the largest American countries, they are not as United as they used to be, and they have never been the largest of the world (that'd be Russia) nor America (that'd be Canada: "second-largest country by total area, with the longest coastline of any country." (Wikipedia)).

Head of America's largest country as of 2026 is King Charles III: https://en.wikipedia.org/wiki/Monarchy_of_Canada