It's not the size of the window but more the direction wind is coming from that could cause issues, as well as how much buffer there is. An awning style cover as Trimurtisan suggested is a good way to buffer some wind while still keeping the window open. I have 3x3 and 2x3 slider windows, none of which face primary wind direction, so I have no issue leaving them open even with rain and snow.
Good way to test for drafts is to go into the coop on a windy day, open all vents, and hold up a lightweight ribbon in areas that the chickens would be - near the floor, on roost bar. Some movement of the ribbon is ok, but if there's areas where it's fluttering noticeably with the wind gusts, that's the type of draft you want to minimize by buffering the vents.
Also the wire on there is not predator/pest proof... I'd suggest adding a layer of wire mesh no larger than 1/2" on top of the wire you have there for extra security against everything from snakes to rats to weasels.