Kimberly:
Your sizes look correct to me, except for that many birds, you only need one nest box. But having them on the outside does no harm, so you can do both with no ill affects.
Aside from that, please don't take this the wrong way, but if you only have a small backyard, where are you going to move it to?
Tractors are moved and moved often as the birds quickly consume......actually, I think they trample as much as they consume or rather foul (fowl?)......but at any rate, the green that was lawn or whatever quickly goes away and is replaced with yucky filth with flies. So you move it to a new place, let the old recover and so it goes.
Do you have enough space to keep moving this to, and all the while the old places are healing and recovering and ready to take their turn again?
If not, perhaps you may want to consider a structure that is more fixed......perhaps a slightly larger coop with run that would sit on the sacrificed area of your backyard? Runs of this type are covered with a roof and the floor is often covered with a deep layer of litter (some are using sand), which can be kept cleaner and more sanitary that dirt alone. Daily, or now and then, depending on a lot of variables, the birds can be let out to roam around in your small backyard.
