Scaling an app to different screen sizes is a bit of a pain - especially when crossing different aspect ratios.
You will need to set your scaling mode to letterbox to ensure everything fits on any screen, but at the cost of having ugly black bars down the sides or along the top and bottom.
I made a small prototype project that allows dynamic scaling to any screen size and aspect, but you have to position all objects in the layout using a set of 4 global variables (the vars change depending on the canvas size, which in turn changes to suit whatever window size is detected).
I'm at work at the moment, so I can't link you to the file in question - but send me a PM if you like and I will try to remember when I get home to send you a link. I did post it somewhere already on the forums here, but I can't remember the thread title! LOL. If I can find it, I will come edit this post and put a link there.
Found it - DOWNLOAD