If you have ever considered building your own chicken coop, you may wonder what the best size is for a chicken door. The short answer is that the dimensions of a chicken door should be 10 inches by 10 inches (10×10).
The Longer Answer To What Size Should A Chicken Door Be?

The size of the door that the chickens use to access a chicken coop (the pop hole door) actually depends on the size of the chickens. Bantam chickens will need a smaller door than full sized breeds of chicken.
The size of the chicken varies even among full size breeds. In my experience, the Cuckoo Maran is a bigger bird than the Russian Orloff.
The truth is that a range of sizes will work for the chicken door. The door should be at least 8 inches wide and up to 12 inches tall. My recommendation comes from experience building a coop and the book Poultry House Construction.
Other Factors In Chicken Door Design
The door should open in a fail proof way. That means once the door is open there is no way for the door to accidentally close. The pop hole door pictured in this post falls open and requires a human to close it.
The chicken door should close securely. I use a spring hook and latch to make sure predators cannot open the chicken door.
Never forget to close the door at night. The one time you forget will be the night that a predator such as a raccoon or possum makes a meal of one of your chickens. I converted my pop hole to a sliding chicken door that closes automatically every night using a timer.
I like your idea of a communal nest box. I had been looking at examples of partitioned nesting boxes. The communal sounds simpler for construction. I think you said your nest box could handle 10 chickens. If you have 20 chickens do you construct two communal boxes? Thanks for your information.
Jim
Jim,
I think that building two nest boxes for 20 hens (each the same size as the one that works for 10) would be a good idea. What ends up happening is that more than one hen goes in the nest box at the same time. Having two nest boxes should provide enough room for hens that want to use the nest box.
-RDG