Some of our departments work with a screen resolution of 800x600. With GuiXT scripting I optimzed the screen layout for this resolution. Now the transactions are also used by other departments who work with a 1024x768 screen size. Is it possible to distinguish these cases in a GuiXT script?

Yes, this is possible as of version 2006 Q3 1. Please use the system variables

V[_screensize_x]   (e.g. 800) 
V[_screensize_y]   (e.g. 600)

Alternatively you can use the window size:

V[_windowsize_x]
V[_windowsize_y]