def set_marksize(self):
        """Set size/radius of marking."""
        try:
            sz = float(self.w.mark_size.get_text())
        except ValueError:
            self.logger.error('Cannot set mark size')
            self.w.mark_size.set_text(str(self.marksize))
        else:
            self.marksize = sz
Set size/radius of marking.