Call this after adding your child widgets.

def apply_saved_layout(self):
        """Call this after adding your child widgets."""
        num_widgets = self.config.get(self.config_key + "/num_widgets", int)
        if num_widgets:
            sizes = []
            for i in range(num_widgets):
                key = "%s/size_%d" % (self.config_key, i)
                size = self.config.get(key, int)
                sizes.append(size)
            self.setSizes(sizes)
            return True
        return False