The necessary registry entries have been made to increase the database size for exchange. However, every hree days the store dismounts and the same error in the application log shows that the database has exceeded the 18 GB size limit.
These are the instructions I followed to create the registry entries:
It appears that the public store size was increased by the registry change, however the private store indicates the database is 23GB in size and the max size it can be is 18GB. I have deleted the registry entries, reboot the server, and created the entries again.