code: @Override
      protected Dimension getDefaultSize() {
          SynthComboBoxRenderer r = new SynthComboBoxRenderer();
          Dimension d = getSizeForComponent(r.getListCellRendererComponent(listBox, " ", -1, false, false));
          return new Dimension(d.width, d.height);
      }
docs: Return the default size of an empty display area of the combo box using
  the current renderer and font.
  
  This method was overridden to use SynthComboBoxRenderer instead of
  DefaultListCellRenderer as the default renderer when calculating the size
  of the combo box. This is used in the case of the combo not having any
  data.
  
  @return the size of an empty display area
  @see #getDisplaySize