The St. John the Divine is the world's largest cathedral, based on the length of its nave. The interior length is 601 feet (187 m). It is "only" the largest cathedral, and not the largest church since several other churches are significantly bigger than this (a cathedral is defined as the official ecclesiastical seat of a bishop). The largest church in the world was the Saint Peter's Basilica in Rome, Italy which was surpassed in 1989 by the completion of the Basilique de Notre Dame de la Paix in Yamoussoukro, Côte d'Ivoire.

The St. John the Divine may also be the world's largest cathedral in terms of interior volume. However, the world's largest cathedral determined by floor space is the Santa Maria de la Sede in Seville, Spain.


Addendum regarding a question from Infinite Burn (and my thanks for the feedback). Besides the Pope's many functions (i.e. the supreme and universal pastorate, Archbishop of the Roman Province, Primate of Italy and the adjacent islands, and sole Patriarch of the Western Church), the Pope is also the Bishop of Rome. However, in this function, his residence is not the Saint Peter's Basilica, but the much smaller St. John in Lateran Cathedral.