@classmethod
def calculate_rtu_frame_size(cls, data):
    """ Calculates the size of the message

        :param data: A buffer containing the data that have been received.
        :returns: The number of bytes in the response.
        """
    hi_byte = data[2]
    lo_byte = data[3]
    return (hi_byte << 16) + lo_byte + 6
