如何改变iPad的iAd横幅视图大小?

I have a SpriteKit game with an iAd Banner in my Main.storyboard file and I've implemented constraints using auto layout to fit the banner to the bottom of the screen. The app is supposed to be a universal app, but when the app is run on the iPad, the banner does not seem to change to the iPad-sized banner, and doesn't fit the whole width of the screen. How do I get the banner to be one size on the iPad and and different size on the iPhone by using only one .storyboard file?I have a SpriteKit game with an iAd Banner in m