GulfstreamGuy is probably correct that the Soviets built the largest single-engine airplane. I don't know of any single bigger than the Ant-25, a little-known research aircraft with remarkable accomplishments.
The Ant-25 was 44 feet long and had a wingspan of 112 feet and a single, 900-horsepower, 12-cylinder engine. Gross weight was 24,750 pounds, of which 13,760 pounds was fuel and oil. It was designed for record-breaking, long-distance flights, and in June 1937 made the first transpolar, intercontinental flight.
With a crew of three headed by Valeriy Chkalov, the Ant-25 took off from Moscow, headed for Oakland, California, via the north pole. Upon reaching the area of Eugene, Oregon, weather and mechanical concerns prompted Chkalov to backtrack to Portland. By the time they neared Portland, word of their imminent arrival had spread, and crowds began to gather at the Portland airport. Remembering the near-riot when Lindbergh landed at LeBourget ten years earlier, Chkalov thought landing at the Army's Pearson Field across the river in Vancouver, Washington, would offer better security. At Pearson the crew was greeted by the Commanding Officer of Vancouver Barracks, Gen. George C. Marshall. The Ant-25 had flown 6,073 statute miles in 63 hours, 17 minutes.
Though little known in the rest of the world, the flight was widely celebrated in the USSR
, and Chkalov was given a hero's welcome on return to Moscow. At Pearson Field, now a general aviation airport (KVUO), there is a stone monument to the flight, and a major street in Vancouver, Washington, is named for Chkalov.
Three weeks after Chkalov's flight, another Ant-25 flew nonstop 7,031 statute miles from Moscow to San Jacinto, California.
Photos of the Ant-25 can be seen at: