Enlarging System -> Window

I want to enlarge the Pointers/Arrows that appear on the top/bottom of my windows.

In mobile these need to be larger to be usable. Any advice?

first try should be changing the window-picture in the system folder of the project - that is where they are defined.
However, that windowskin has only a limited space assigned to how the arrows should look like - you can check the help file for more details.
If you want those arrows to be bigger than the maximum possible by windowskin, then you'll need a plugin to get the arrows to use another source.