import javax.swing.plaf.basic.BasicComboBoxRenderer;

static void setComboBoxListFontSize(final JComboBox cb, final int fontSize) swing {
  cb.setRenderer(new BasicComboBoxRenderer {
    new DefaultListCellRenderer defaultRenderer;

    public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
      JLabel renderer = cast defaultRenderer.getListCellRendererComponent(list, value, index, isSelected,cellHasFocus);
      setFontSize(renderer, fontSize);
      ret renderer;
    }
  });
}

Began life as a copy of #1012966