GUI positions are absolute meaning that they will change their relative position if the screen size changes. To make them relative you need to calculate your Rectangles using factors. eg.:

void OnGUI () {
  public float x, y, width, height;
  Rect pos;
  pos.x = x * Screen.width;
  pos.y = y * Screen.height;
  pos.width = width * Screen.width;
  pos.height = height * Screen.height;

  GUI.Button(pos, "test");
}

With x, y, width and height being values between 0...1 this should lead to relative positioning.