I will have a shed roof as well so there will be plenty of ventilation at the top and eaves. There will also be some overhang, but even with overhang, wind can blow rain sideways as well as we all know. Especially during a good Nor'easter here in Maine.
I guess I am realizing it probably doesn't make much difference.
One thing to consider is how it will be held open. If hinges are at the bottom, I can just let it hang straight down if I want it all the way open, or use a chain and hook to keep it partially open. On the other hand, if it is hinged at the top, I will need some way to keep it opened or propped open... I guess it will be a last minute decision.
As far as my plexi windows go, here's what I did. I did not want to cut the plexi so I approached it this way... for an 18" x 24" piece of plexi, I framed my opening to be 19" x 25". This allows a half inch all the way around the plexi when the window is shut flush. To create the window frame, I made the outer dimensions of the window frame 4" greater than the plexi size or 22" x 28". I then cut an opening using a jigsaw that was 3" in on all sides or 16" x 22". This opening size allows for one inch overlap of the plexi on the inside for fastening to the plywood (I used 19/32" plywood)... Anyway - I'm sure this is very hard to follow on a forum. I hope it made some sense. Oh yes, don't forget to seal/caulk in, out and around...
Steve