a. Do you have any 'top tips' on special needs for a group of this size?
As Lisa says, the wall is really important and I leave it chaotic with post-its on one side and room for the folks at the wall on the other.
b. Any special suggestions for how to create the circle/s? (I am considering three concentric circles: one of cushions, one of benches, and then one of chairs. But we could also possibly build risers. How much space will we need for this circle/these circles? Do you suggest three concentric circles or more?
With 600 I go with more concentric circles. I find that when the centre circle is at or over 150 people, then the connection with the sponsor and the opening is less. I prefer a centre circle of between 100 and 150. A small platform, one step up at the most, in the centre of the circle might help but it must be easily gotten on and off of. I would not build fixed seating in risers.
c. How much extra time will be needed for the agenda setting, given the size of the group?
I have not experienced that a lot of extra time is needed – say .5hr.
d. How many sessions should I expect from 600 people? (In a highly motivated group of 100 people, I would usually provide breakout spaces for about 50 sessions. But my guess is that this ratio should go down as the numbers increase. Also, we expect a lot of people who are too shy, reserved, or not sufficiently motivated -- this time -- to post a session. So I am guessing that I'll need about 150 breakout slots.
I plan for .7 times the number, say about 42 per round and then have extras available, so that is similar to your number for 100. I've never had need for 150 breakout slots in a given round of OST discussions with 600. You know your people better than I and the theme for this event. The amount of passion evoked by the theme question shapes the number at any given round. You are talking between 3-4 rounds of discussion.
e. Any advice on the schedule? The day is limited to 8.30 – 16.30. Some people might be able to stay later, but we can't plan on this. I normally don't schedule tea breaks, but in this case, it might be necessary to give people time to find their way around the venue. Here are three versions:
I would go with the simplest agenda and 1.5 hour session. With this number, there will be more time for folks to move between sessions so allowing 1.5 hours for each will give them the freedom to self-manage that time – which for me is the genius of OST. Building in breaks makes it seem like any other conference and reduces the "whenever its over, its over" injunction.