Howdy! Welcome to Permies!
Okay, there aren't any hard and fast rules for this because each individual situation is very different.
It all depends on climate, rainfall, where your coop ends up in relation to shade and
water use/placement, et cetera.
You want to make sure there's easy air exchange going on, but not a direct breeze on the birds while roosting.
The placement of the "windows" is entirely dependent on too many factors for the rest of us to guess.
According to my husband (who loves playing with fluid dynamics, and considers air just a really thin liquid), having two windows/spaces opposite each other in a way that catches the prevailing wind and prevents a draft along the birds roosting space is probably the easiest thing. Like opening windows on opposite sides of your house allows the air to circulate much better than having two windows open on the same side. If one of the windows happens to be in the same direction as the prevailing winds, even better.
If you don't know which direction the wind comes from, putting a window space on every wall to allow for air flow would work. You could always cover the spaces not needed later if weather, air temperature, or other conditions needed it.
Without knowing what your conditions are, I can't really tell you firm numbers. You want to have enough air flow (however you get it) to allow for trapped moisture to evaporate and not too much air flow that would make it uncomfortable for your birds. Too much air flow is as potentially dangerous as too little (she says with a nearly completely open-air roosting space that has tarps over it and on the tops of the "walls" for protection from direct wind and rain).
If you need hard numbers, here's some I'm completely pulling out of the air - 4 inches or 10 cm tall and a good 8 -12 inches or 20-30 cm space on each wall to allow for good air flow. Or whatever can be covered by an easily divisible amount of hardware cloth or pre-made vent cover spaced at reasonable distance along each wall.
Or put a 4 inch/10 cm band of hardware cloth along the top, right under the roof of your coop, making sure there's no direct air movement along the
chicken roosting space.
One thing I have done is put a tarp up that covered all of an open wall expect for underneath the bird's roosting area. There was no water or air that was on the birds and they were able to make it through both excessive heat and excessive cold just as easily.
We can't tell you what no one knows. We can only suggest and advise that, as long as there's a reasonable turn over of air and there isn't a buildup of moisture and there's no draft on the birds, whatever works for you is fine. Better than fine. It's perfect for you and your situation.
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