I have a large app that I will need some time to optimize for iOS9.
Edit: What I am worried about is all the UI getting squeezed together when the app window size is reduced. So my question is, is there any way to force full screen for the app?
To opt-out (disable) multi-tasking for your application:
Select your Target → General Section → Scroll Down and check Requires full screen
It gets applied to the plist's UIRequiresFullScreen
key value.
Note: Apps are required to support all screen orientations if it supports multitasking. Otherwise, apps will get the following error on publishing:
Invalid Bundle. iPad Multitasking support requires these orientations: ...