Ventilation that you'll be using in cool/cold weather (i.e. all year-round) should be high up above chicken level, at the tops of the walls, ideally protected from rain and wind to some degree by roof overhangs. You can put vent slots, long and relatively narrow, atop all four walls. (By narrow I mean like 4"-8" wide or something like that, not an inch or two width of 'arrow slot', unless it is a small coop for just a couple few chickens.) Vents near the roost are good in hot weather but bad in cold weather. Ask yourself "will a chicken experience a noticeable breeze on the roost in the winter?" If yes, arrange things so you can shut down those vents when temperatures drop.