i dont think size really matters for them they will lay were they feel the most comfy , my little bantams either lay in the enclosed section of the rabbit hutch or within the already built in nest area of thier house thats about 2 foot long and about 10 inches width i think .pretty much anything can be used as a nest box just be carefull that its not so lightweight that it can be toppled over and cover the bird i made that mistake a few years ago with large plastic toy trays when i got home in the afternoon i found 2 very stressed sweaty hens stuck under one .