The audit database is not created until you enable auditing. By default, auditing is disabled.
The initial storage requirement for the audit database is 100 MB for the database and 80 MB for the transaction log.
You can enable auditing for individual Enterprise Vault servers. The auditing events for several Enterprise Vault servers in a site can be written to a single auditing database.
The amount of space required will depend on the number and type of events logged and the level of detail required.
The Utilities manual describes how to set up auditing.
Limit transaction logs to an appropriate size for your back-up and maintenance plan. For instructions on how to roll over the audit database, see this Symantec Support document: