I've 7 boxes, four 12" wide by 16" deep, three 16" x 16". Fake eggs in one of each. Almost 20 layers presently (waiting on the days to lengthen some). Have never found two birds in one nest, though I understand it to be common at times. Also, have never noticed any relationship between box size and bird size. They are like high school kids. They form cliques and establish their favored hang outs. Its a lizard brain thing.