A room which has space around the table in which to move about, but not one which makes the group feel small in comparison, is ideal. Comfortable chairs and tables coupled with refreshments on a nearby table are useful. Providing an object in the middle of the circle gives people something to fix on while thinking and removes the need to look into the face of someone else while suggesting an idea. This releases some pressure on the facilitator who can spend more time guiding the process. Smaller groups are easier to control but there are less people to keep the process moving smoothly onward.
Advanced techniques are very useful to kick start the flow of ideas.
A very small group is more like a quick-fire conversation and could be seated round a small table with a large pad of paper covering the whole table surface. Everyone can add their ideas at the same time. In this situation you will need to have a theatre-style seating pattern with the facilitator at the front. If you need a microphone and speakers then it's likely that the group is too big for brainstorming because ideas will be lost while people wait for their turn to speak. If you want to brainstorm with such a large group then you need to have everyone write their ideas down on a notepad or on a computer, use some ideas as stimuli to help people with their personal brainstorming and then gather the pads in afterwards.