pfsense log file retention

There are several ways to do this. Why don't you read the excellent and useful mailing archives for pfsense, or check their forums?

Anyway, there are two ways to increase logs. First, you can increase the size of the clog files by re-initializing them. Another way is to install a regular syslogger which captures logs in the regular way. You can then use that syslogger to forward logs to a central point. If you are in a secure environment where you have to guarantee to retain all logs, then having the clog + local syslog + remote syslog is best.

and for the syslog-ng.conf: http://forum.pfsense.org/index.php/topic,7793.0.html