First, check the specs of your refrigerator if it's not built-in/integrated. Ours needed 37" x 25" (which luckily is what we built to, since other models we were considering needed that too). I'd get 25" deep panels if you are planning a counter-depth freestanding model.

I'd match the height of the over fridge cabinet to the height of the adjacent uppers, if this is correctly showing that the adjacent uppers are 20" high over open shelves/appliances. We have a similar setup and I put a shelf over the refrigerator below the upper so that there wasn't just a lot of filler--it has worked out very well. It's adjustable so that if we get a refrigerator that needs a different height, we can move the shelf up or down without modifying the upper cabinet. It's currently used for cookbooks but could be used for cookie sheets if it needed to move higher.

using an awning cabinet over a fridge isn't that great of an idea - you might get it opened without standing on a chair, but how do you get it closed?

You would need a stepladder for any cabinet at that height (unless you're tall!) We use that space for longer term storage of items we only use a few times a year, but in a small kitchen it's still much needed. I also prefer the finished look of a cabinet there vs. keeping it open.

Thank you everyone. The 20" leaves about 6" to the top of the current fridge, and every fridge that I have considered is a similar size. And as one of you pointed out, the 20" pairs with the neighboring cabinet. I have two doors. I can go doors, or tie them together and go awning. But it seems that doors make sense.

options to fill the 5” gap gap, https://www.pinterest.ca/madelinebeaumon/vent-above-fridge/

We are oversizing the fridge cabinet from 36" to 37" because you never know which fridge you will want in the future, and many 36" require a larger opening. I've never had a fridge that will not fit under a 20" (although I know that some built in options are taller). We do not have a dedicated designer.