I would like to know as well!! anybody knows??

Took me a while to find it...  the settings are under the Admin menu, Event Viewer.  Then there is an Edit Log Settings link below the Event Viewer.

Each type of event log message has a limit you can set to the number of messages.  My problem was there were a few event log types that had no limit.  However, I still needed to go into the database and manually delete the messages because the EventLog table had expanded to the point that DotNetNuke could not delete the messages without timing out.

The message limit for each type of event log message appears to be invoked each time that you view the Event Viewer.

The last trick was that there is a catch all category.  There was no limit on that in my install, however, loggin was disabled.  Even so, I had a number event log messages that had been assigned to that category, probably from a few versions past.  I had to enable the catch all category so I could set a message limit.  Then I disabled the catch all category and the limit was applied to the event log.