Size of TN DB

We are currently logging all data relating to documents passing through TN in Oracle. This does cause a significant growth ratio, why we are in the process of eliminating, as a minimum, storage of bizdoc content where not needed.
Our main concern is to keep TN fast (we do in some cases have to retrieve data from previously received massages and process these), as well as not to run out of space on the DB.
Does anyone have any recommendations as to what the maximum size of the TN database should be before it might affect the performance?