What kind of chickens do you have? Obviously, a huge heavy standard sized hen will need more room than a small, light bantam . . .
Our plans for our coop call for nest boxes that are 12" deep. I suppose if you wanted you could make them anywhere from 12"-18", though -- how big is the interior of your coop? While researching chicken coops myself, I noticed that most people's nest boxes are 12" deep or thereabouts, some were 12" square, some were 12"x15" (like ours, so 12" deep x 15" across, so a total of 30" for 2 boxes), some were 12"x16", but most people kept their boxes around 12".
This seems like a very reasonable space to me, as a matter of fact, I was surprised after we cut the lumber ourselves for our own boxes, at how large the boxes actually were. Ours wound up being 24" high, so the total dimensions were 24"Hx12"Dx15"W, with a pretty steep pitch on the roof to keep the chickens from trying to sleep up there (our breed apparently likes to roost! We'll give them a 2x4 to roost on instead, LOL).
Oh, we also cut a 4" high strip for the front of the next boxes, which is a tad higher than is typical, because we're going to use stuff for bedding that can be easily tossed or kicked out, and we don't want to lose our next box bedding all the time (I'm allergic to both hay and straw, so it's newspaper shreds or pine shavings for us!). Hence the 4" high strip, in the hopes that it will be high enough to keep the bedding in, but low enough for the chicken's use.
Does that help?
Whitewater (new to this myself!)