I struggle to shrink some interfaces to fit the full range of devices, is there a case structure to separate the iPhone sizes?

You mean this?

No, but that is really useful also.

I mean how do I know if I am on an iPhone 5 or iPhone 8?

Or do you not worry about what the phone type is and just check for screen size?
Select case screensize
case 3.5"
case 4"
case 4.7"…

This is how to check the size of the screen (in points)