6,000 would be the sweet spot. That's about where St. John's and Villanova are on campus. Really, this is the type of game that should be played on-campus, with major non-conference and weekend conference games being played at The Rock.
I do think that the school should at least explore the possibility of a public-private partnership with Essex County for a renovation of South Mountain Arena or building a 5000-10000 seat arena somewhere else, ala what
RU is doing with Middlesex County for baseball. I can't think of another county that has been as active in building parks and attractions than Essex County, it's not a totally unrealistic scenario to think they might want to build their own small arena like Trenton has.