You cannot change that value. The database schema is limited to 20 Bytes. See 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)
)
If you need just longer slugs in your URLs you can do that in the argument rewrite
:
register_post_type(
'yxz123',
array (
'rewrite' => array (
'slug' => _x( 'really-long-slug', 'slug', 'custom_plugin_textdomain' ),
'with_front' => FALSE
),
)
);