不能更改该值。数据库架构限制为20个字节。看见wp-admin/includes/schema.php:

TABLE $wpdb->posts (
  ID bigint(20) unsigned NOT NULL auto_increment,
  post_author bigint(20) unsigned NOT NULL default \'0\',
  post_date datetime NOT NULL default \'0000-00-00 00:00:00\',
  post_date_gmt datetime NOT NULL default \'0000-00-00 00:00:00\',
  post_content longtext NOT NULL,
  post_title text NOT NULL,
  post_excerpt text NOT NULL,
  post_status varchar(20) NOT NULL default \'publish\',
  comment_status varchar(20) NOT NULL default \'open\',
  ping_status varchar(20) NOT NULL default \'open\',
  post_password varchar(20) NOT NULL default \'\',
  post_name varchar(200) NOT NULL default \'\',
  to_ping text NOT NULL,
  pinged text NOT NULL,
  post_modified datetime NOT NULL default \'0000-00-00 00:00:00\',
  post_modified_gmt datetime NOT NULL default \'0000-00-00 00:00:00\',
  post_content_filtered longtext NOT NULL,
  post_parent bigint(20) unsigned NOT NULL default \'0\',
  guid varchar(255) NOT NULL default \'\',
  menu_order int(11) NOT NULL default \'0\',
  post_type varchar(20) NOT NULL default \'post\',
  post_mime_type varchar(100) NOT NULL default \'\',
  comment_count bigint(20) NOT NULL default \'0\',
  PRIMARY KEY  (ID),
  KEY post_name (post_name),
  KEY type_status_date (post_type,post_status,post_date,ID),
  KEY post_parent (post_parent),
  KEY post_author (post_author)
)

register_post_type( 
    \'yxz123\', 
    array (
        \'rewrite\' => array (
            \'slug\'       => _x( \'really-long-slug\', \'slug\', \'custom_plugin_textdomain\' ),
            \'with_front\' => FALSE
        ),
    ) 
);