What size lensboard would you put on a 20 x 24 inch camera? This is a wet plate camera, so take into consideration the size of very early lenses.
I am in the drawing stages and right now am considering 14", but I think that may be a little overkill.
I have a 10" square lensboard on my 11x14 Century 8A studio camera and think it can handle any lens I can afford. :) The Dallmeyer flanges top out at an ID of ~ 7", so if you find that 8D at a yard sale--you'd still be ok with a 10" square board. In the extremely unlikely event you can find a 30" f/3.8 petzval, it would still fit on a 12" lensboard. You've gotta ask yourself a question: "Do I feel lucky?" Well, do ya, punk? :)
On my 20 x 24 Ebony I have 9 inch lens boards. My Dallmeyer 8D that I found inside a little ol' ladies car that had never been driven fits inside of it--but just barely. It weighs 18 pounds and is does well with the 9 inch boards--aluminum though for the strength. I would not use wood at 9 inches with that lens as I wouldn't trust the wood. Barry is right 10 inch would cover just about everything you could throw on it in terms of the diameter but if you are going with wood instead of aluminum then you might consider the bigger sizes. It is my opinion that your gut about 14 being too much is accurate. If you and your buyer are set on wood then I would get the densest wood I could find to handle the bigger lenses. I think just as important if not more is the strength of your hardware when it comes to holding the lens. My camera will rack out to over 6 feet worth of bellows and when you get it out there even half that distance the front standard should have strong metal on it. Even then I use another tripod under the big lenses. Just my experience.