Interesting question. Are we talking about the volume or the floor area? For volume maybe a church? Then St. Peter’s Basilica in Vatican City would be the largest. I don’t know the layout though, but I assume a large portion of it is the main “room”.
Or do stadium with a roof count? Then maybe one of these?
Edit: I don’t think I really thought this through. I was thinking too much of more roomy rooms. Most convention centers probably have larger exhibition halls.
Well that sure is a bit larger than a church.