You can specify the maximum size for the security log, and what happens when that size is reached, by following the directions in the "Event Viewer" chapter of the Windows NT Workstation or Windows NT Server System Guide.
One of the regular tasks of network administration is examining the security log to track significant events and monitor system usage, and clearing the log as necessary. It is recommended that you routinely archive the log before clearing it.
In additon to the "Event Viewer" chapter in the Windows NT Workstation or Windows NT Server System Guide, Chapter 2, "Windows NT Security Model," of the Windows NT Resource Guide provides information on viewing and managing the security log. If you need more detailed information, see the Appendix D, "Security In a Software Development Environment."