I would cut two windows, one each on opposite sides of the coop. You want cross-ventilation to get good airflow. But be sure these are above the chickens' heads where they roost. You don't want a draft on them, where airflow will disturb their feathers. The feathers protect them and keep them warm, you don't want their skin exposed. You also don't want them getting wet. Take measurements then go to your hardware store and buy a louvered vent, then go home and cut the opening to fit. Hopefully these will be big enough to allow light as well as ventilation.
Can you post a picture of your coop?