Limit size of cachetags table
So it appears that Drupal core neither limits the size of cachetags table and not it provides a mechanism to limit it via a setting, like it offers for other cache bins.
Cachetags table is not cleared on cache reset, neither via web browser no via drush.
There is another issue noted in Drupal core that cachetags table can grow endlessly as has been the case that I have come across.
So a blunt method or periodically cleaning the cachetags need to be performed. Something like follows with drush or direct sql statement via mysql cli client or php-myadmin.
drush sqlq &quot;TRUNCATE cachetags;&quot; -l &lt;uri&gt; --no-interaction