Get the actual size of the content, as some attributes have variable sizes

def _len_objid(self):
    '''Get the actual size of the content, as some attributes have variable sizes'''
    try:
        return self._size
    except AttributeError:
        temp = (self.object_id, self.birth_vol_id, self.birth_object_id, self.birth_domain_id)
        self._size = sum([ObjectID._UUID_SIZE for data in temp if data is not None])
        return self._size