It is common to collect AIX logs.
The primary syslog configuration file is syslog.conf. to see how to config the syslog.
man syslogd
It is hard to say how large your log should be.
Ours is only 5M (5 log files together).
Anyway Don't letlog files take over 5% of your disk space, that will slow your system a lot.
To configure the size of the log, start smit (type smit at command line)
go to "Problem Determination", goto "Error Log" or "Alog" then "Change/Show Characteristics of an Alog File", press [F4], choose the log you want to modify, then you see the log size setting.
If you wang secadmin to be able to read logs, make sure this user account have read permition on the log files (try su to this account and read the log file). Use smit to read logs.
Some logs are binary files so you need smit to read them. In smit, go to "Problem Determination", goto "Alog" then "Show an ALog File"; or goto "Error Log" to generate a error report.
We do have a script to archive the logs bi-weekly and we keep the log for at least a year (on tapes/CDRs).
The following is a sample syslog.conf you can copy and paste to your syslog.conf
auth.notice /var/adm/authlog
kern.debug /var/adm/messages
*.emerg;*.alert /var/adm/messages
*.crit;*.warning /var/adm/messages
*.err;*.notice /var/adm/messages
*.info /var/adm/messages