I can't answer all your questions, but here are a few:
They should not be pooping in them, ( an occasional time will happen)
Hardware cloth is NOT in my opinion the good floor material. Wood would be better.
Mine have wood floors. SOme put a soft rubber mat laid in over the wood.
I have never seen one of my roosters in a nestbox. Sometimes more than one hen, but no roos.
Slightly smaller is better than too big, as hens like some privacy while laying. Larger breeds may need up to 16 inch dimensions, but most breeds do just fine with a 12" x 12" in nest or 12' by 14".
As to materials, my hens like straw, but I don't use it much as it makes a good place for mites. A small bug. SO I use pine shavings/bedding.
Hope some of this helps,
I think on my BYC homepage their is a pic or two of exterior and interior of nestboxes.
Dave