Xcode 6 Do I have to make an application for every screen size?

So I have created my webapplication and when I build it in xcode I have to under "Simulated Metrics" > "Size" set it to 3.5 inch since I have a iPhone 4s. When I build it and run on the iPhone everything looks perfect, but I want this application to run on iPhone 5/6 aswell, but when I change the size to something else in the Simulated Metrics it gets really messed up on my iPhone. I'm using a webview of my responsive website which shouldn't really care about the screensize but I believe Xcode does. So, is there a solution where xcode automatically detects screensize and makes the webview take the whole viewcontroller? Or do I have to create a application for every screensize?