Xcode 6 Storyboard the wrong size?
While Asif Bilal's answer is a simpler solution that doesn't involve Size Classes (which were introduced in iOS 8.) it is strongly recommended you to get used to size classes as they are the future, and you will eventually jump in anyway at some point."
You probably haven't added the layout constraints.
Select your label, tap the layout constraints button on the bottom:
On that menu add width and height (it should NOT be the same as mine) by checking their checkbox and click add constraints. Then Control-drag your label to your main view, and then when you de-click, you should have the options to center horizontally and vertically in container. Add both, and you should be set up.
If you are using Xcode 6 and designing for iOS 8, none of these solutions are correct. To get your iPhone-only views to be sized correctly, don't turn off size classes, don't turn off inferred metrics, and don't set constraints (yet). Instead, use the size class control, which is an easy to miss text button at the bottom of Interface Builder that initially reads "wAny hAny".
Click the button, and choose Compact Width, Regular Height. This resize your views and cover all iPhone portrait orientations. Apple's docs here: https://developer.apple.com/library/ios/recipes/xcode_help-IB_adaptive_sizes/chapters/SelectingASizeClass.html or search on "Selecting a Size Class in Interface Builder"
Do the following steps to resolve the issue
In Storyboard, select any view, then go to the File inspector. Uncheck the "Use Size Classes", you will ask to keep size class data for: iPhone/iPad. And then Click the "Disable Size Classes" button. Doing this will make the storyboard's view size with selected device.
In Storyboard, select your ViewController and go to Atribute Inspector. At the very top, under Simulated Metrics you have Size and Orientation properties which are set to Inferred. Change them to desired values.
In order for an application to display properly on another screen size, you also have to setup constraints, as described by Can Poyrazoğlu in the first post.