Sometimes, it's convenient to have a long or wide view controller in Storyboard when you're designing long or wide views. If that's the case, configure the view controller to be freeform.
In the Storyboard, make sure the view controller is selected, then click the Ruler icon, select freeform, and choose your dimensions.
At runtime, the app will attempt to shrink the view controller back down to a normal phone size. Sometimes, you want the subviews to shrink with it, and sometimes you want it to stay the same size.
For example, if it contains a scrollview, you probably want the scrollview to shrink down. In that case, set the autoresizing mask to have a flexible height and flexible width, as shown below. Tap the arrows in order to highlight them red.
For a fixed size UIImageView, you probably don't want the view to change dimensions. In that case, unselect the flexible height and width arrows.