I have two regular double hung windows approx 24x36 on the east and west side of my coop that I can open to provide ventalation. This give the coop plenty of light, and I have the option of any amount of venting. I covered the window opening on the inside with 1/2" hardware wire so nothing can get in the open windows. In the summer, both windows are wide open. In the winter, I keep the protected western window (open out to the covered outdoor run) about half way open and control the amount of air moving through the coop with the other window. During snow storms, I close the eastern window, or I will be shoveling out my coop! Looks for windows on Craigslist - you can often get them for free.