Open Space events have been held in "perfect" rooms with a great amount of flexibility for creating an "ideal" site. They have also been held in "challenging" rooms with more rigid setups, such as a site where all the chairs are bolted down and facing forward. They have been held in rooms where you cannot post anything at all on the walls, and they have been held outside. In each case, space can be opened by a flexible facilitator who holds intent and creates and holds space for her or his group of participants.

For the purposes of this paper, I will combine various facilitators' words and ideas to explain the sort of indoor site that you and your client/sponsor may find for the "ideal" site.

One Large Room

As Open Space is a process of bringing "the full system into the room," it is ideal to hold the event in one main room large enough to seat all participants in one circle of chairs (with as much space/air between each chair as possible); if you need two or more rows, add aisles and give as much room between one row and the next as possible (the idea is that any participant wherever they are sitting will feel the freedom to walk through the other seated participants to get to the center of the circle for posting their topics). There should be no tables in the center of the room, and ideally no pillars. You will also want to create enough room around outside of circle for participants to be able to freely walk up to and post things on the walls; especially the agenda wall. My own preference is to have one room which will hold the opening circle, most discussion areas, food and beverage tables against a wall, computer/newsroom tables (if used) against another wall -- all in the same room to provide energy, activity and opportunities for conversations to occur as people move to or past different. If break-out (additional discussion group) areas are added, best to have them next door, outside, and otherwise in close proximity. The idea is for participants to be in main room as much as possible where everything is happening. As they go for a nibble they drift past a great discussion and join in, for example.

How Large Should the Room Be?

Sites often list the capacity of their meeting rooms for a setup in theatre style, U-shape or rectangle. But usually they have never heard of a circle with no tables. So you will have to calculate the maximum circle size of the room yourself. In order to provide enough room for an Open Space Technology meeting, you should secure a room that holds double your expected number of participants, theatre style seating (although your Open Space will not be in theatre style seating). For example, if 60 are expected, look for a seating capacity of 120. Keep in mind that this works best for a room that is more or less square, or round. You want to set up a circle or concentric circles of chairs, not an oval, if possible.

Koos de Heer has created a simple spreadsheet that gives a somewhat more precise calculation, if you like -- and it shows you how many chairs you will want to place in the opening circle, or concentric circles. Take the width of the room (the smallest side if it is a rectangle). If the room is 30 by 40 feet, the 30 feet is the measurement you calculate with to get the capacity of a nice circle. Any space left over at the end of the room can be used for the agenda wall, for a buffet or for one or more break out groups. The spreadsheet below can calculate the capacity of up to 3 concentric circles. And apart from the main room, you can also calculate the capacity of up to three break out spaces (discussion areas separate from the main room). The tabs in the bottom let you switch between metric and imperial measurements (meters and feet). This is a very user-friendly tool - you can try out what two concentric circles would look like, or three, or one -- the spreadsheet does all the work for you!

He offers this Excel spreadsheet as is and for no charge, and you are using it at your own risk.
http://www.auryn.nl/ost_room_size_calculation.xls
If you have any questions using it, or want to suggest improvements, you are welcome to email Koos at openspace@auryn.nl.