It really depends on you and what your planning on doing. For example I built a 40x50 run/pen for my birds with a 12x16 coop from the beginning and easily housed about 2 dozen laying hens and a hand full of roosters (now that count is up to about 50 hens). However Im fortunate enough to not worry about many predators so 99.9% of the time the gate to the run is open and I enjoy seeing them out in the yard. I have expanded that original coop to now 21x28 with 3 separate rooms, still the original 12 x16 for my layers but added a 12x21 room for growing out chicks and hanging cages for my exhibition birds, and the other room in front is utilized for breeding pens and another spot for growing out more chicks.
Again it depends on why your getting chickens, how much time you have for them, and forewarning you the hobby is quite additive so build what you can afford. Best of Luck!