After countless tries I figured I'd try the duh one for kicks and giggles, figuring it'd absolutely not work because it's too simple.... Yeah it worked.

ItemRect.x = #;

That'll change the x location of your button. You can use that for scaling the objects as well.

Edit: More code for ya since you asked.

Rect myPosition = new Rect(0,0,1,1);
void OnGUI()
{
     if(GUI.Button(myPosition), "Move me")
     {
          myPosition = new Rect(10,10,10,10);
     }
}