What venue is the largest indoor theatre in the world?
Answer: Radio City Music Hall.
I'll be honest, I'm not 100% sure how they're qualifying this. I think it's either by square footage or by venue genre. I say this because Radio City Music Hall's capacity is right around 6,000. The Philippine Arena in the Philippines, on the other hand, holds 55,000. Here's a list of other theatres that hold way more people. I say "venue genre" because even Wikipedia qualifies their list as indoor "arenas". Now, maybe there is a difference between arenas and theatres, but in mind they achieve relatively the same thing. Regardless, Radio City Music Hall is an iconic landmark in one of the greatest American cities, so I'm going to give them the benefit of the doubt.
The background is also incredible too. It was opened at the height of the great depression by one of the most famous business magnates ever to walk American soil. Read more here.
If you don't believe me, check out Radio City Music Hall's website.
"Radio City Music Hall is the largest indoor theatre in the world. Its marquee is a full city-block long. Its auditorium measures 160 feet from back to stage and the ceiling reaches a height of 84 feet. The walls and ceiling are formed by a series of sweeping arches that define a splendid and immense curving space. Choral staircases rise up the sides toward the back wall. Actors can enter there to bring live action right into the house. There are no columns to obstruct views. Three shallow mezzanines provide comfortable seating without looming over the rear Orchestra section below. The result is that every seat in Radio City Music Hall is a good seat."