I have to say that designing for screen sizes and resolutions that you can't/aren't going to actually test for is a mistake. You need to test it on the devices you are targeting. Would you trust a car manufacturer that has never driven their car on the road?
There is no one size fits all. The past has always been about designing for the lowest common denominator, if you don't want to do that, then pick a size that you feel works for your game. Test how it scales both up and down and make your decisions based off of testing, not opinions from the rest of us who don't have your game and can't test it on multiple devices.
If you want to seriously do well as a game designer you need to test these scenarios thoroughly and adjust your design as needed. You can't just rely on other saying well this size may work for your game which we have no idea how it scales on different devices.