If we are using storyboards we can handle different sizes by creating different storyboards for iPhone 4 and iPhone 5 and handle them in didFinishLaunching method of AppDelegate in this way :

Here for iPhone 4 we have storyboard named "Main_iPhone_4s" and for iPhone 5 we have storyboard named "Main_iPhone"

UIViewController *initialViewController;

    CGSize iOSDeviceScreenSize = [[UIScreen mainScreen] bounds].size;

    // iPhone 4
    if (iOSDeviceScreenSize.height == 480)
    {
        // Instantiate iPhone 4 storyboard
        UIStoryboard *iPhone35Storyboard = [UIStoryboard storyboardWithName:@"Main_iPhone_4s" bundle:nil];

        // Instantiate the initial view controller object from the storyboard
        initialViewController = [iPhone35Storyboard instantiateInitialViewController];
    }

    // iPhone 5
    if (iOSDeviceScreenSize.height == 568)
    {
        // Instantiate iPhone 5 storyboard
        UIStoryboard *iPhone4Storyboard = [UIStoryboard storyboardWithName:@"Main_iPhone" bundle:nil];

         // Instantiate the initial view controller object from the storyboard
        initialViewController = [iPhone4Storyboard instantiateInitialViewController];
    }

    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.rootViewController  = initialViewController;
    [self.window makeKeyAndVisible];