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.