I've read the documentation about designing for different screen sizes and I'm still lost. How can I design an application that will work for iPhone 4, 5, and 6 screen sizes?
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The easiest option is to use "letterbox scale" fullscreen mode and it will work on any size screen at all. If the aspect ratio is different, it'll show black bars down the sides (like 16:9 movies on a 4:3 TV).