Mark, where did you hear this? Do you have a link so I could read it in context? Perhaps there is some specific situation where this matters, but I've seen hens make nests in some pretty small areas and some really open areas. The tiny Seramas can make a nest in something much smaller than a huge Jersey Giant. Many nests don't even have a depth as they don't have a top.
You often see a recommendation for 12" x 12" as the minimum size for a nest. People need guidelines so they need some numbers. There is nothing magical about these numbers but a nest this size will be big enough for any breed of hen to lay and should be big enough that a broody hen can hatch eggs in it. Most breeds can lay in something smaller. Many of us use larger nests quite successfully. One community nest box design is 2' x 4' and is supposed to handle 24 hens a day, but they are not good for a broody hen.