Mitsu-san There are a number of pages on The Egyptian Hall in Eddie Dawes book--'The Great Illusionists'; but canot see anything about capacity. It does mention the replica of Seti's Tomb & other exhibits which means the Large Hall mus have been pretty big.
When we were researching together, a couple of years ago, I did find info at a source in Sydney, Australia. The Sydney Library I think ??
I know there was an actual seating plan. If I still have it on the PC or in my files--I will look later and send it direct to you