How to change window size for various OS - like iOS

I have some application that works and looks nice on the Macbook and it doesn’t fit the screen, it has const size of window, and it’s super for me, but on the iPhone it looks ugly. Is there any way to change window size to make it automatically fit to the iPhone screen?
I probably should use something like #if and #endif or elseif. But actually don’t know how it works and how to make it to recognise various OS? Could anyone give some hint?

Have a look at the tutorials, especially this one.