Answer: The official Vatican guidebook gives the dimensions of St. Peter's Basilica:

"The first impression one has on entering St Peter's is one of vastness. In fact, it is the world's largest church. On the floor, along the axis of the nave, a metal inscription records the length of the largest Catholic and non-Catholic churches in the world. None are as long as St Peter's, which (including the Walls), measures192.76m. The total distance across is 58m.

Source: Guide To The Vatican Museums And City (1996)

One can look around the internet and get several different measurements. However, on the surface, St. Peter's does hold 1st place.


St. Peter's Basilica
Scott 32-34 (1933)