"Largest" is at best a vague term, which is often not qualified by claimants. Accepted measures of largeness could include area, volume, length, width, height, and capacity (although the latter is far more subjective). It is important to note therefore that churches may claim to be "the largest" based on only one of these measurements; and thus that there may be several churches have equal claim to be the "nth largest church". Since there is no official body governing these claims, there is no generally accepted criteria for being "the largest church".
St. Peter's Basilica is World famous and every one knows about it so I will not cover it here but will cover above three churches one by one in my subsequent posts (not necessarily consecutively). I also seek readers comments/opinion regarding this.