NetSight Linux syslog log rotation is controlled by the "logrotate" utility on the box. By default the /etc/logrotate.d/rsyslog is set to rotate daily.
If you want to modify this file you can set it to "weekly" "yearly" or you can set a specified size that is used to determine when the log is rotated with the "size" command.
The /etc/logrotate.d/rsyslog file does not look to be rewritten on upgrade of the software, so the modifications should be permanent.
For more information on options for the logrotate utility:
http://www.linuxcommand.org/man_pages/logrotate8.html
This is a linux server correct?
Thanks
-Ryan