It was about 5000 or more people, my Jazz band was opening for Chuck Berry and Bo Diddely at an outdoor summer concert in Worcestor MA, summer of '89.
In 2001 I played piano for Rich Hebert, a leading man of the Broadway stage in a Stars to the Rescue Benefit at the Orpheum Theatre in Phoenix AZ. The other artists on the program included Huey Lewis, Wynnona Judd, Bruce Hornsby (who graciously allowed Rich and I to use his own Baldwin 9ft Concert Grand) and comics Kevin Nealon and Elaine Boosler. The place was packed, must have been about 2000 people in there. BTW Bruce Hornsby is a pianist who has carved out his own thing, most people know him primarily as the last pianist/keyboardist for the Grateful Dead, but he is an astonishing improviser; at this benefit he went out and just started to play, free improv a la Keith Jarret, but more interesting; this ran the gamut from rock, jazz, Asian (?) music, Hindemith, Ravel, all sorts of influences and things he had absorbed coming back at the audience after having been filtered through him. One of the coolest piano performances I have ever heard, better than most mainstream Jazz pianists. If you ever have the opputunity to see him play solo, GO. You'll be glad you did, probably.
When I was working as a pianist/conductor/musical director in Musical Theatre audiences of 2000 to 3000 nightly were not uncommon, but I was in the pit and not the focus of the audiences attention .
My biggest classical audiences have numbered from about 700 to about 2000...