title: how to get the default size of a window in pyqt4
body: <p>When I create a <code>QMainWindow</code> without explicitly specifying its dimensions, PyQt will give it a -let's say- "standard size" which is not the minimum that the window can get.</p>
  
  <p>Can I set this size at will in any way?</p>
  
  <p>My goal is to get this "standard size" according to the currently visible widgets, when I set the visibility of some widgets on/off.</p>