Different builds for different screens in apple store iOS

Hello, this is a bit advanced question about iOS development in Unity. I use 2DToolkit, it's an asset that saves time and handle GUI and sprites. In 2DToolkit I can add to the game different sizes for graphics to support different screen sizes (1x, 2x, and 4x). When the game start I detect the screen size and use the x size suitable. But this caused the game to occupy big size on devices. I read somewhere that app-store can support different builds for different screen sizes, but the complicated guides I found were not for Unity. Anyone has an idea how to that for Unity? I can easily create 3 different versions of the game, one for each screen but how do I set app-store to detect the user's screen and deliver to him/her the version with the right size? Thanks