Open Space: how large a meeting room do we need?
Accomodations 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'll have to calculate the maximum circle size of the room yourself. Rule of thumb is the size in theatre style divided by two. Keep in mind that this works best for a room that is more or less square. Otherwise you might end up with an oval and that is not as nice as a circle. I have created a simple spreadsheet that gives a somewhat more precise calculation. 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 (if it is a lot) 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. The tabs in the bottom let you switch between metric and imperial measurements.
I offer this spreadsheet as is and for no charge, and you are using it at your own risk. If you have any questions using it, or want to suggest improvements, you are welcome to email me at firstname.lastname@example.org. Have fun with it!