def format_file_size(size):
    """
    format the file size as string

    @param      size        numeric value
    @return                 string (something + unit)
    """
    if size >= 2 ** 30:
        size = size / 2 ** 30
        return '%1.2f Gb' % size
    elif size >= 2 ** 20:
        size = size / 2 ** 20
        return '%1.2f Mb' % size
    elif size >= 2 ** 10:
        size = size / 2 ** 10
        return '%1.2f Kb' % size
    else:
        return '%d' % size
