If you define stadium as being enclosed by a roof, than the largest stadium in the world is the Rungrado May Day Stadium in Pyongyang, North Korea. It is a multiuse facility that was completed on May 1, 1989. Many Communist rallies are held there and a gigantic birthday celebration for Kim Jong-il’s 60th birthday in 2002, which was one of the few public events in North Korea to allow foreigners to attend. U.S. Secretary of State Madeline Albright was entertained at the May Day Stadium upon her visit in 2000. She was the highest ranking U.S. official ever to visit North Korea.
If, in your world, a stadium doesn’t need a roof, then the world’s largest sports venue is the Indianapolis Motor Speedway. At full capacity, including the infield, this venue can hold up to 400,000 people. That’s a lot of race fans! It was opened on August 12, 1909, and obviously is mainly concerned with car racing, but it hosted the opening ceremonies for the Pan American Games in 1987.