I think you will find 14X14 is pretty "standard" but a quick check of the inside size will give you a good idea. And then it gets into what you want, a vent cover can get you the cover which lifts up to open of some mean the cover on top of the cover which lets you leave the first cover open to get air but the top cover keeps rain out!
This is the one we favor for the second type:
https://www.walmart.com/ip/MaxxAir-0...YaAgheEALw_wcB
This one is larger and lets the first cover stay in place and then this fits over and around.
But shopping will get you lots of options for pretty much the same.
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Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!