We are getting 3 Black Australorps in exactly 4 weeks from today (yay!) and our plans called for 2 nest boxes for a total of 30" long, with the boxes themselves being approximately 12" deep by 15" across. We used 1" plywood (that we painted with a wood stain from Behr) so the actual dimensions probably vary a bit. The nest boxes are also about 18" high, which looks a bit high to me, with a fairly steep angle on the roof, to keep them from roosting there, supposedly -- we'll see what our girls think. The farm that we are getting our chickens from has nest boxes that are pretty much 12"x12" and their birds seem to do just fine -- some adult Australorps were in them laying when we went up to choose our youngsters.
I'm getting the feeling, from reading everybody else's posts, that no matter how well-built your boxes are (they could be perfect, ideal specimens!) your hens will have a mind of their own about it, and you'll never be able to please them! Again, I'll tell you a couple months from now when ours start laying whether or not this is true, LOL.
Whitewater