How to test different physical sizes in the Game tab?

I can easily change the screen resolution being tested in the Game Tab, but I cannot set the Physical Size. I.e. It is impossible for me to set this up as if it were a 2560x1440 resolution screen that is only 5.1 inches in size. Is there an option to turn this on? or perhaps an asset for it?

In the drop down of the screen resolution (default Free Aspect) there should be a little + icon at the bottom. You can setup your own custom resolutions there and it will stretch it to fit into the game window.