I once used a kitty litter bucket that measured 7-1/2" x 11-1/2" for large fowl. It was OK for them laying but did not work out well for hatching. There was no top so unlimited height. Hens stand up when they lay so a little height is good, but when I was a kid on the farm I saw some hens lay in some pretty tight spots. We give a lot of rules and such but not all hens read our rules.
A general recommended minimum size is 12" x 12" x 12" for large fowl, which sounds reasonable to me. I made the ones I built 16" x 16" x 16" which worked well. The main reason I went 16" wide was that my wall stud spacing was 16" so that made framing easy. When you cut an 8' long piece of lumber or a 8' x 4' sheet of plywood into 16" pieces there is no waste. !6" for all dimensions seemed reasonable for me. I often had three hens sharing the nest at one time. There was enough room. If you are using left over materials look at what sizes you have and plan accordingly.
You can build a roll-out nest, there are various designs for those. The hen lays in a nest but the egg rolls to a separate box where you can collect it, which could be outside the coop. Those are not good for a broody hen hatching eggs but could work for you.
Some people build community nests, maybe 24" x 48" with holes at both ends for entrance. The top is hinged to gather eggs. One this size is supposed to handle two dozen hens. Some people use long troughs uncovered.
Some people like to build dark nests, maybe even putting curtains on the opening to keep them dark. Others do not put any covers on them at all, leaving them exposed to light.
Some people give rules for how many hens can use one nest. I do that myself, but size of the nests make a difference. The larger the nest the more hens a nest can handle. If your nests are 12" x 12" x 12", I'd suggest one nest for every four hens. My 16"ers regularly handle 5 hens each and could probably handle more.
There are no hard and fast rules for what you have to do, your hens can't read them. But be generous with room. Just like practically everything else with chickens and room, it's a lot better to provide a bit too much than a bit too little.
