For coop size, plan a minimum of 2 square feet per bird, but better to have 3-4 square foot per bird and for outdoors a minimum of 3-4 square feet per bird. If they will live confined, as mine do, add extra outdoor playroom area.
The main mistake all us newbies make is underestimating the grown size of the birds and how much room they really need. The imagery I like is to use a 5 gal bucket to represent each chicken, mark off the area you have and plan and also include space taken up by water fount, feeder and chicken ramp…and see how crowded it can look.