'Returns (size, num_objects)
Raises ASFHeaderError'

@classmethod
def parse_size(cls, fileobj):

    header = fileobj.read(30)
    if ((len(header) != 30) or (header[:16] != HeaderObject.GUID)):
        raise ASFHeaderError('Not    an    ASF    file.')
    return struct.unpack('<QL', header[16:28])
