The Beaumont could well be right. The August Wilson is very deep because it was originally built by The Theatre Guild to hang multiple plays in repertory.
I'd dare to guess the Broadway Theater or the Gershwin?
"People have their opinions and that doesn't mean that their opinions are wrong or right. I just take it with a grain of salt because opinions are like as*holes, everyone has one".
-Felicia Finley-
I may be wrong, but I believe tge Gershwin had the biggest onstage space. But the Beaumont Theatre has the biggest stage, including backstage and the wings. The wings in the Beaumont are enormous!
I do recall hearing that the Majestic, Gershwin and old Mark Hellinger (why not still count it) have the largest BACKstage spaces. And that the continuing occupancy of the first two make it hard for really big shows to find spaces.
If anyone feels like diggining through theatre specs, Shubert lists them on their site http://shubert.nyc/theatres/. At quick glance, I think the Broadway takes the largest proscenium opening and depth of their theatres