My screen size code right now is pretty simple.
.: In CREATE Event :.
viewWidth = 1920 /4;
viewHeight = 1080/4;
windowScale = 4;
var horSize = viewWidth * windowScale;
var verSize = viewHeight * windowScale;
window_set_size(horSize,verSize);
//then i set the surface size to the same values,
surface_resize(application_surface,horSize,verSize);
And then lastly i center the window to the center of your screen.
So there is no fullscreen mode when you start the game!
How small is your screen?
Mayeb i can add a "windowed size " property in that case where you have the options x1, x2, x3, x4 etc