Criteria
What a dialogue forum must not do
The temptation of a space with access to decision-makers is to use it to ask for something. That's where these forums go wrong.
A forum that seats decision-makers and those affected by those decisions at the same table carries an obvious risk, and it's best said before someone else says it.
Asking for something
The temptation is to use the access to raise a personal matter. Once that happens once, everyone else stops speaking freely: they now know what the room is really for.
That's why personal matters have no place here. Whoever has one has other channels, and those are the right ones.
Mistaking presence for endorsement
Someone attending doesn't mean they endorse anything said there, nor that the forum backs their position. It's stated at every gathering because it's worth repeating.
Growing
A forum that admits everyone who asks stops being a forum within two years: it becomes an auditorium, and in an auditorium nobody has a conversation.
Size is part of the method, not a limitation to overcome.