I'd say the largest subway station in the world is the one that is also a train station, the busiest train station in the world. Of course it's Shinjuku station in Tokyo. The underground passages extend from the Shinjuku Washington Hotel on the west side to Shinjuku san=chome station on the east side, a distance that surely approaches 4 km. The station includes 3 different rail companies (4, if you include the nearby Seibu Shinjuku station) and 3 subway lines (also 4 if you include walking underground to access the Fukutoshin line). There's also a long distance bus station that is part of the structure.