def sizeHint(self, option, index):
        box = QtGui.QStyleOptionButton()
        style = QtGui.QApplication.instance().style()
        return style.sizeFromContents(QtGui.QStyle.CT_CheckBox, box,
                                      QtCore.QSize(), None)
Reimplemented to provide size hint based on a checkbox
dataset/ETHPy150Open enthought/traitsui/traitsui/qt4/extra/checkbox_renderer.py/CheckboxRenderer.sizeHint