There are a lot of different screen sizes and a few solutions
My favorite approach is to put everything into a safe zone (scale outer), but that only works for certain kind of games.
Most important if you can't go for safe zone you have to decide wich aspect ratio.
Important to read
https://www.scirra.com/tutorials/73/sup ... reen-sizes
https://www.scirra.com/tutorials/1126/m ... le-devices
Here are some numbers you have to know.
Iphone 4 960x640 3:2 (this is the ideal starting point, becaue smallest for safezone)
Iphone 5 1136x640 16:9
Ipad 1024x768 4:3
the real 16:9 HD 1280x720 amd 1920x1080
and of course a lot of 16:10 Displays and some 5:3