Deeper boxes are more popular with my
chickens than shallow ones - our deeper ones are 14 inches from front to back but just less than 11 inches across the front.
Dark-coloured boxes also appeal to my hens. I got some rejected paint for cheap that could realistically be called, "dark, puke green". The chickens like it just fine!
Some commercial boxes I was given, are as wide as they are deep, and a light grey colour and some chickens will use them, but most will sneak into the duck nest boxes in the same area which are made out of 1/4 of a black barrel with a
wood front attached - dark, deep, and private!
Lots of places say 1 box/5 birds but from my
experience, the birds will jam into their favorite box and we end up with 10 eggs in one, 5 in the next, and none in the last.
On the human side of the equation, I insist on nest boxes that are easy to remove from the coop for cleaning and that either the back or the roof can be easily removed to access and see better while cleaning. Older hens are more likely to lay a weak-shelled egg which can easily break and make a mess. Wild birds can bring mites or lice to your flock. There will be situations where you need to thoroughly clean and decontaminate a coop, or at least the nest boxes, so if I'm looking at coop designs, that's the first thing I consider. Three days in bright sunshine will sterilize a lot of things as well as, or better than any chemical from my experience. (And not hurt my lungs or give me a rash!)