Different screen parameters

[ldries46 <bertus.dries@planet.nl>]

I am using a high definition screen 4K (3840 * 2160 pixels) and two 
computers one with a graphic card which can handle this resolution and 
an older one which can handle up to 1920 * 1080. (the first on Windows 
10 the second on Linux Ubuntu). I want both computers capable of running 
my programs even if I define a the window on 2000 * 1200 for a Gtk 
project. On the computer that can handle the resolution the there is of 
course no problem. On the other the screen is window is to big for the 
resolution and is shown only partly (of course).

My question is:
Is there an Ada library that is able to somehow read the  maximum sizes 
of the screen in pixels so I can firstly maximize my values for size 
definition in Gtk.

I already tried:
Maximize(Mainwindow);
Get_Size(MainWindow, Height, Width);

The result was 200 * 200