In answer to your questions, I think the biggest crowd we played for when I was with the Trio was in Kansas City at their annual summer festival. We played on a stage at the end of a long downtown street that was packed with people as far as we could see. Estimated crowd was 10,000. Do you know how many people can pack into Fenway Park? That was a big one too when we sang there for a Red Sox game.
Smallest crowd? Maybe in our back yard for Meri's 50th birthday party, but even so we had over 100 people there.
As for the same questions for the Bros4, you'll have to excuse my affliction with "Part-timers". My memory of specifics from those days is a bit foggy, and to be honest, I don't know that we were often informed of crowd size. I do remember one tour that we did for Roy Raden out of Long Island. He had us touring military bases in Texas and at this one base we had an afternoon show at a NCO club. Apparently the show was only promoted with our group name, "The Brothers Four". When we came on stage we looked out on a room filled with African-American service men. After the first song the room pretty much cleared out, as the audience realized that we weren't the "brothers" they were expecting to see. We played the rest of the show to 5 or 6 white guys. That would be the smallest audience.