I think your confusing what the recommendation is.
For iOS 9 applications, when creating a UIWindow in code, it is recommended that you use -init, as that method will create a window that is full sized for your application's current multitasking bounds. This however does not extend to UIView – -[UIView init] creates a zero sized view. If you are subclassing UIView (which it sounds like your latter comments say) then you need to override the designated initializers, -initWithFrame: and -initWithCoder:.
However, to answer the original question, the likely answer is you don't have the correct launch images. A new project in Xcode will configure correctly for all of this and should always give you a fullscreen window at the device's natural size. If you are updating a current application, you will likely want to emulate what Xcode does for its default projects.