Add to /opt/zimbra/bin/zmlogrotate and/or /etc/logrotate.d/zimbra
It is a normal logrotate style file. You can add entries for logger_myslow.log. I included a wide varity of options. More of a discription can be found at man logrotate.
There is also a file /etc/logrotate.d/zimbra that is the same as zmlogrotate. I'm not sure which one to edit. I know logrotate will rotate the one in /etc/logrotate.d for sure though. To manually rotate the logs you can use logrotate /opt/zimbra/bin/zmlogrotate. I guess one could symlink one to the other to simplfy things.
create 0660 zimbra zimbra