My nest boxes are just a bit smaller than 12x12 but are 18" high. I made them that high for the hens to stand up in there and to accommodate a thick layer of nest material. They have been just fine for all my hens from the tiny Sebright to my Cuckoo Marans who weighs just over 8#.
Normal size hen.
Hen with 2 of her half grown chicks in with her.
Sebright hen and rooster in nest.
Normal size hen.
Hen with 2 of her half grown chicks in with her.
Sebright hen and rooster in nest.