ideal opening size for nest boxes?

[stitch81]

I purchased some nest boxes to get us through the winter, since we didn't have time to build the external boxes for our coop. My concern is that the openings of the purchased boxes are too small. I look at my chickens and look at the opening and wonder how they are going to fit in there. What is the ideal opening size for your nest boxes? I have barred rocks, buff orpingtons, rir, standard size chickens....

 

[Ridgerunner]

I've had full sized fowl (Orpington, Delaware, Australorp, Sussex) use an opening 6" high and 8" wide, so they can use something that size or maybe smaller.

 

[TwoCrows]

I have two nest boxes for my very large Australorp girls. One box has a 14 inch opening and the other has a 9 inch opening. Half of them prefer the tiny box. Not only is the opening small, but the entire box is way to small for them. But they stuff themselves in it anyways.

Edit to add: Put in some fake eggs in the new boxes. That will tempt them to use them.