But in line with ridge runner's point the featherbrain link doesn't list the size of the chickens that is considering.
A standard-sized chicken can be thought of as a 15" cube -- about a foot for the actual chicken plus a few inches of personal space.

@aart has a useful article on heights: https://www.backyardchickens.com/articles/coop-stack-up-how-high-stuff-works-well.73427/