Fiddler, you can calculate the size of the needed baffle by using the quarter wave formula.
It depends on how deep in the bass you want these things to produce.
Pick a frequency above the F3(resonant freq) of the driver, and deep enough into the bass region to satisfy you.
For example, if you want 32Hz to be able to be reproduced by the system(assuming the driver goes that low), you will need a baffle that is at least 1/4 wavelength of the (32')32Hz wavelength, which would be 8 feet across. If you want to get only to 64Hz(16' wavelength), then you'd only need a baffle that is 4 feet across. If you want to "split the difference, and get 48Hz, then you could use a baffle that is 6 feet across.
All frequencies above the frequency determined by the baffle board will be reinforced by the boundary effects of the baffle board, and will not be subject(much) to the "wrap-around" cancellation effects that will adversely affect the frequencies below the capability of the baffle board to handle.
While it is a matter of discussion whether to mount the driver in the center of the baffle board, due to the relative "smoothness" of the response curve, mounting it in the center would give the best result in getting to the deepest bass for a given size baffle board.
The determining factor(in size) for the baffle board's effectiveness is based upon the smallest outside dimension that goes across the driver. So the smallest outside dimension will dictate your reinforcement frequency, and any larger dimensions will have little effect, but they may do a little. Thus, a circle shape will be the minimum sized shape that you could use, but it is more difficult to stand up and use. A square baffle is typical, but only the largest diameter circle that you can draw around the driver will be doing the work, and the corners doing very little.
Remember, if you make it tall and narrow, you are losing your effectiveness, so make it just as wide as it is tall.
Having the piano hinges, and angling the sides backwards to a small degree will do just as well, and helps to make the baffle more visually acceptable. It also keeps the edges out of the diffraction plane.
The best thing would be to make the whole baffle shape like one of those "snow saucers". It would allow the driver to be placed at the foremost part(center), and have uniform shallow radius sloping backward all the way around, for most smooth response and minimum diffraction. But it would need to be big, and I've never seen a 6 foot snow saucer!
For the easiest construction, flat and square works.
