In my experience, all of the UI that I've created with NGUI have been fixed width, unless I specifically anchor (UIAnchor.cs) any parts of the HUD to a corner of the screen. So all my iPad, iPhone 4, iPhone 5, and Android ports get the same dimensions for the UI layout (so it sometimes looks "squished" in the middle for some devices like the iPad, and then as far too much screen real estate on other devices like the iPhone 5).
What are you seeing when you create your UI that it's NOT giving you a fixed width?
