St. John the Divine is the largest cathedral in the world.
It's 601 feet long indoors (over 2 football fields in length). To get some perspective, look at the size of the doors on the front of the cathedral. Those doors are at least 2 or 3 people-heights tall.
Those little dots at the bottom are people. I've been in there. It really is that fricking big. Restores your faith.
St. Patrick's Cathedral is the largest gothic catholic cathedral in the US and is the seat of the Archbishop of NY.
It's around 400 feet long and seats 2,200.
Not quite as big as St. John's, but still not bad.