SpriteKit - How do I handle multiple screen sizes?

My new game is finished and now I'm just testing on multiple real devices. I came across multiple issues after testing. The big issue is how to handle screen sizes. The game is how I want it to look on 6 Plus/6s Plus, but not on 6s, 6, 5s, 5, 4s, 4, or iPad.

I found these two answers but I don't know how to implement them: How to support multiple screen sizes in SpriteKit? and SpriteKit how to get correct screen size

I really would like any type of help, because this is irritating me.