GUI Screen.width & Screen.height example

function OnGUI () {
 GUI.Box (Rect (0,0,100,50), "Top-left");
 GUI.Box (Rect (Screen.width - 100,0,100,50), "Top-right");
 GUI.Box (Rect (0,Screen.height - 50,100,50), "Bottom-left");
 GUI.Box (Rect (Screen.width - 100,Screen.height - 50,100,50), "Bottom-right");
}