I have discovered the origin of this notion. In D'Arcy W. Thompson's 'Wayside Thoughts', on a page quoted in the Christ's Hospital Book, p.236, there occurs the sentence: 'Our dining-hall was, with the exception of Westminster-hall, the largest room in Great Britain unsupported by pillars.'Martin wrote:About 50 years ago, I think that the Dining Hall was the second largest such building. Westminster Hall, in the Houses of Parliament was then the largest. Perhaps it’s still true.Kit Bartlett wrote: It was such a huge area; someone once told me that it was one of the largest buildings in the country unsupported by pillars.
Thompson's book, though, was published in 1868, so the reference is clearly to the hall in London (presumably the 'new', Victorian, hall).
The Hall at Horsham must be at least as large as, and probably larger than, than the one in London was (it had to accommodate the Hertford boys). But there are probably larger unsupported halls by now.