Want to see a glimpse of what the General Assembly Open Space process might look like? See this day-long Open Space meeting condensed to thirty seconds.
Of course, GA is longer, bigger and more complex, but the point is the same.
I think the point is that participants self-select which break out into the groups they want, but there are maximum room sizes at GA and the ushers will be there to insure no group is too large.
A one day Open Space at the Tate Modern, viewed by a time lapse camera. (YouTube)