This could help you : https://www.scirra.com/tutorials/73/sup ... reen-sizes
In my own case, I ended up having to choose a layout size and make sure that important elements (sprites, text, not background) were centered enough and a bit far from the edges, to make sure they'd show well on any resolution. In one case I programmed the game to adjust an image's size depending on the device used (the player tells which device he has when the game starts).
You can also play around with the Anchor behaviour, but the more you add elements that you want to keep on-screen, the harder it gets : https://www.scirra.com/manual/88/anchor
In the end, if your question is what aspect ration (width/height) you should choose, my answer would be to choose an average one (say, the average between the most rectangular and the most square screens you're expecting your game to be played on).
Still, I'm new at designing games for mobile devices too, so this is only what I'd do.