The CITCON Europe 2008 conference this weekend has been my 3rd open space conference. It was also the largest so far. I wasn’t able to get as much out of it as last time and partly I think this had to do with the size. More participants meant more proposed topics (ca. 60), while at the same time the available space and time slots for topics did not increase (5 x 5). This resulted in most topics being moved around a lot and being combined with others in one time slot. According to the open space principles the person who suggested a topic is responsible to steer the talk but once topics are being combined, it is then up to the topic owners to combine and coordinate their talks, often without any time to prepare.
Also many talks were quiet full, which according to my experience doesn’t contribute to deep discussions and exchange of thoughts. In large groups typically some and often the same people stand out and dominate the discussion with their opinion, while in a smaller group it is usually more balanced.
So my conclusion is, that also here less is more. A smaller conference might make it more difficult to find sponsors but on the other hand costs would also be much lower. The alternative could be to expand the conference in space and time but then costs would go up. Personally I would rather have a small conference each quarter than one or two large ones per year. 50-60 seems to be a good number of participants for open space conferences.