def _get_size_url(**kwargs):
    """Get url for QCEW data by size.

    Note that size data is only available for the first quarter of each year
    Parameters
    ----------
    year : string
        'YYY'.
    size : string
        For all establishment size classes and titles see:
        http://www.bls.gov/cew/doc/titles/size/size_titles.htm

    Returns
    -------
    string
        URL that will be used for main function.

    """
    for arg in ['year', 'size']:
        if arg not in kwargs:
            raise ValueError('Must specify {}'.format(arg))
    url = 'http://data.bls.gov/cew/data/api/[YEAR]/1/size/[SIZE].csv'
    url = url.replace('[YEAR]', kwargs.get('year'))
    url = url.replace('[SIZE]', kwargs.get('size'))
    return url
