You can use the bounds property on an instance of UIScreen:
CGRect screenBound = [[UIScreen mainScreen] bounds];
CGSize screenSize = screenBound.size;
CGFloat screenWidth = screenSize.width;
CGFloat screenHeight = screenSize.height;
Most people will want the main screen; if you have a device attached to a TV, you may instead want to iterate over UIScreens and get the bounds for each.
More info at the UIScreen class docs.