I say thanks for posting too! I have from bantams to light brahmas in one pen and was wondering! Edited to correct my silly fingers' idea of spelling.
You can make a 8'' X 8'' six sided box with a 4.5'' hole cut into the front and that will work for a nest box on some bantams. I currently have Stilt Legged Moderns that use this size nest box. You will need to make one and watch to see if it is working for you or not. 12'' X 12'' will work even for most full sized breeds.
