I have been to the odd very small one, say 5 singers, harder work but decidedly rewarding.

I have also been to a few whoppers where once around took all afternoon and, despite a quite high standard of performance I didn't feel the energy level was the same. There just seemed less joy in participation, though of course that might have been down to the personalities of a different group of people.

My favourite would be around 10-14 people, which gives you something like a song per hour, depending on people's choice of song, and on how much comment goes between them. It allows for a decent spread of differing tastes and levels of ability, but is small enough to keep people attentive.

Splitting up into smaller groups can work if the premises suit, but I would be cautious around splitting on grounds of ability as so many people have a mismatch between ego and ability. I would hate to be the person doing the grading, but would also doubtless disagree with another judge's views. Self-grading would certainly be revelatory, but would not really work, IMO.