def key_period_size(period):
    """
    Defines a key in order to sort periods by length. It uses two aspects : first unit then size

    :param period: an OpenFisca period
    :return: a string

    >>> key_period_size(period('2014'))
    '2_1'
    >>> key_period_size(period('2013'))
    '2_1'
    >>> key_period_size(period('2014-01'))
    '1_1'

    """

    unit, start, size = period

    return '{}_{}'.format(unit_weight(unit), size)