I think conference rooms might be on the endangered species list because of all the alternatives. But let’s say you are past that discussion, and you need to size a conference room.
The first thing to remember is that conference rooms are always too small. If you get the size correct, your conference room will be a more valuable asset. The two main factors in sizing a conference room are number of attendees and the function of the room. Since not every group needs their own conference room, part of the sizing process is to determine what everyone's needs are.
While you are surveying everyone's needs, determine the function of the room, too. The three main functions are discussions, presentations and teleconferencing. If discussions are the main function, then the size depends on the table and appropriate clearances for chairs. These clearances should be 42" in most cases. 36" is adequate for six people or less, and 48" is desirable for 10 or more people. For more than 24 people, it may be desirable to place a bank of chairs behind those seated at the table. In this case 84" is the ideal distance from table to wall.
Next consider the table size and shape. Round tables are good for discussion-type meetings of six or less. Rectangular tables are best for presentations and any group over six. Allow two feet of table edge per person along the sides and add one person at each end. The table width should be at least 36"; and 42" or 48" is better for groups of eight or more. For groups of 12 or more consider a boat-shaped table (wider in the middle) or increase the table width to 48 or 54 inches.
If the function of your conference room includes presentations, then you should increase the chair clearance at the presenter's end to a total distance of 84 inches or more up to 120". For presentation-type conference rooms the door location becomes important. Place the door in one of the corners away from the presentation area and reserve the walls at the ends of the table for displays. Presentations will work better without windows to outdoors, which is true of almost any conference room.
Teleconferencing is beyond the scope of this article because the system selected becomes a major factor in the size of the room. The market for these systems is shrinking because of the prevalence of inexpensive or free alternatives: GoToMeeting, join.me, Google Hangout, Apple’s FaceTime, Skype.
For the other types of conference room, you should be well on your way to determining the size that is right for your needs. Start with the table, add clearances for chairs and presenting. Add a door. Consider accessibility, which would add, at least, part of a five foot diameter circle near the room entrance.
Remember, when in doubt, make it larger. In operation, a too small conference room can be a problem; too large never is.