The size of sessions will always be a problem if they get too big. Luckily this is self-regulating, generally; people just stop coming, and maybe someone starts a rival so it splits like an amoeba.
    Physically, a session can't work if it is too big. People just can't hear each other, so the timing goes. There is also a question of acoustic physics: we all know that the speed of sound is finite, that if we see a batsman hit the ball there is then a time delay before we hear the impact. Just how this effects music is not always apparent to people, but it is an appreciable effect across a big stage, or across a big room. Twenty feet apart, people can't coordinate fast rhymic music. That is why you need monitors in front of you on a big stage, so that you hear your fellow musicians when they hit the notes, not a fraction of a second later. Same thing applies in a big room. Sound goes at 1000ish feet per second. That means it takes 1/50th of a second to cross 20ft of room. When you're playing a fairly fast fiddle tune, you are churning notes out at about ten per second.So the person at the other end of the room is hearing you a fifth of a note late.So if they plays what they think is in time with you, when their sound comes back to you, it is 2/5 of a note(nearly a half) later than when you played it. So it's hardly surprising that rhythmic precision is not possible at a big session. That's why big orchestras need conductors, they play in time with the beat given visually by a central person.
    It is perfectly satisfactory in a teaching situation to sit at the centre of a horseshoe of 30 fiddlers and share a tune. But I don't think that number of people can sit and pleasurably play tunes together in a big room in a pub. The rhythm will never be good enough for people to get off on the swing.
(Sorry, Will Fly, this has diverged from a discussion of "correct" guitar chords).