While length, height, wingspan and empty weight are all valid criteria in ranking aircraft, the one most often used to determine which plane is biggest is the maximum takeoff weight (MTOW). That's the most a plane can weigh and still get off the ground; it incorporates not only the weight of the plane but also its fuel supply and cargo (human or otherwise).
Using the MTOW criteria, it's not hard to pick a winner. It's the Antonov An-225.
The An-225 is a cargo jet. It transports vehicles, supplies and all sorts of aid all over the world. But it began its life serving a different purpose. It was built to haul around the Buran, Russia's space shuttle. When the Russian space program was discontinued in the early '90s, the An-225 found itself unemployed and went into storage.
Years later, one of the two An-225s in existence found a new role as a commercial cargo jet (the second remains in storage to this day). It's ideally suited to the role because of both its size and its maximum takeoff weight. It can accommodate 46,000 cubic feet (1,300 cubic meters) of cargo. That's enough for five military tanks or eight double-decker buses [source: BC]. More important, with a maximum takeoff weight of 1.32 million pounds (600,000 kilograms), it can get those tanks or buses off the ground -- and fly them almost 3,000 miles (4,800 kilometers) to their destination [source: Aerospace, Eisenstein].
For comparison, both the Boeing 747 and the An-124 (the model the An-225 was based on) have maximum takeoff weights of 900,000 pounds (405,000 kilograms) [source: Aerospace]. The A380 can get off the ground carrying about 1.24 million pounds (560,000 kilograms), which is a pretty close second and beats out any other passenger aircraft [source: GA]. The massive Spruce Goose had an MTOW of 400,000 pounds (181,000 kilograms), which is still the record for a seaplane [source: Aerospace].
While the An-225 tops all aircraft, and the A380 beats out every other passenger plane, the future of aircraft design could make going even bigger a little easier. Increasingly lightweight composite materials are allowing for more size without more weight, and advances in aviation technology and flight design are making it easier to get more weight off the ground. With the A380 in service as of 2007, we can no doubt look forward to seeing progressively massive airliners in development in coming years [source: FG].
For more information on aircraft and the An-225, including complete specifications, look over the links on the next page.