choose a preset size for the plot
        :param size: 'small' for documents or 'large' for presentations

def set_size(self, size):
        """ choose a preset size for the plot
        :param size: 'small' for documents or 'large' for presentations
        """
        if size == 'small':
            self._set_size_small()
        elif size == 'large':
            self._set_size_large()
        else:
            raise ValueError('Size must be large or small')