At my huuge congregation of 1,565 members, a "quorum" is 50 -- as long as fifty members sign in at a Parish Meeting, all the decisions made are valid. Fifty has been the quorum for the last 60 years at least, while the congregation has grown steadily from 175 all the way up to 1500 adult book-signed members.
Well, I decided that it is high time to have a discussion about increasing the quorum or, even better, making the number a percentage of our membership. I floated the idea of 10% -- 150 members. Someone immediately mentioned that we would never be able to make a decision if we required that many people! In reality, ever since 300 of us voted to fund our building addition, we've easily had 150 and more at our Parish Meetings. Part of this is just getting smart about scheduling a PM: serving lunch after the second service, then going right into the meeting. None of this 5 pm Sunday-potluck-and-meeting crap. Hardly anyone went to those; in fact, several members of the Board of Trustees didn't even attend.
What do other very large congregations do about this? Many of the decisions at my congregation aren't even put before the membership because we have policy governance. As a matter of fact, our mission changed last year, after needing to be improved for many years. Finally, a whole group of lay leaders collectively brought up the issue of revising our mission two years ago, but it was changed by one person: our senior minister. That's right: he re-wrote it by himself. Mission accomplished!
We members still call ministers and vote for the yearly slate of officers, council chairs and committee chairs. And we would be the ones to amend the by-laws and the Bond of Union.