The database is SQLite. The size of the database can go to a terabyte or more however you have to recognize that the more data you put in your system the more memory is necessary to retain speed.
The speed of your system is far more dependent on your processor and RAM than it is on the SQLite database.
You may want to set a policy for purging calls every 3-4 months to avoid carrying data that is no longer necessary.
Regards,
Administrator