def _find_fixed_instance_size(oneandone_conn, fixed_instance_size):
    """
    Validates the fixed instance size exists by ID or name.
    Return the instance size ID.
    """
    for _fixed_instance_size in oneandone_conn.fixed_server_flavors():
        if fixed_instance_size in (_fixed_instance_size['id'],
                                   _fixed_instance_size['name']):
            return _fixed_instance_size['id']