Hello all. Something like this tutorial may already exist but, in designing my game, "Psi," I've come across an easy way to support any size screen and aspect ratio.
1. In "project settings," set "window size" to something with a 16x9 aspect ratio. An easy resolution would be 1600x900.
2. In your project properties, set "fullscreen in browser" to "scale inner."
3. Make sure to keep crucial assets towards the middle of the screen so they won't be cropped on screens with a narrower aspect ratio.
Now, on a 16x9 aspect ratio screen, you will see everything and on narrower screens the edges will be cropped without cutting anything vertically.
I hope this is helpful! Good luck and happy game making!