If you want to add iPhone 5 dimension remember to update to the newest iOS 6 sdk. When you compile you might run into this error first, Xcode Missing Retina 4 Launch Image.
To enable your apps to work with iPhone 5, you need to add a retina version of the launcher image. It should be named Default-568h@2x.png. Add a new launch image named Default-568h@2x.png to your project and it will work!