How to Support All iPhone Screen Sizes

I have an app in Xcode 6.2 with the storyboard supporting all devices. For some reason, whenever I load the application on the iPhone 6+, or iPhone 4s, or iPad, the screen doesn't take the size of the iPhone/iPad. However, on an iPhone 6, 5s, and 5, the screen takes the full size of the iPhone. Why is this? I am very unfamiliar with sizes and how to properly support all sizes. Right now, I have set the screen sizes in the storyboard for each view controller to 320 x 568. The width is "Compact" and the height is "Any".

If this has anything to do with this, my iPad screen comes out blank. It's just white. Apple recently rejected my external testers app submission due to this.

I have done lots of research on these 2 questions, but I had a hard time understanding anything, as I am new to iOS Development.