如何在GDK中获得不包含Unity侧面板的屏幕尺寸

I'm trying to make Guake terminal work correctly in Unity. Its window have width that is equal to screen width. But because of Unity left bar window's right border becomes invisible. So, I want to set proper width for window. It must be smaller than actual window size. And the code must work correctly with or without Unity.I'm trying to make Guake terminal work correctl