ahh, ok you need some proper advice from mobile developer then,
I have not made for mobile
I assume to go full screen properly on all devices will require more than simple scale option.
(sorry edited a bit)
I assume you will need to use crop or centered
also you will need to move hud items by using anchor behaviour and maybe 9 patch for border items etc.
you may also need to manage the scale for different resolution/ dpi screens not sure about this but maybe