I agree with Tal, it is extremely unlikely deleting /var/log/messages files and elg files will free much space. Even with extreme levels of logging, those files collectively should never take up even a whole gigabyte.
Instead, look in $FWDIR/log at firewall logs. This is the common set of files per day of log data:
2023-11-25_000000.adtlog
2023-11-25_000000.adtlogaccount_ptr
2023-11-25_000000.adtloginitial_ptr
2023-11-25_000000.adtlogptr
2023-11-25_000000.log
2023-11-25_000000.logaccount_ptr
2023-11-25_000000.loginitial_ptr
2023-11-25_000000.logptr
They may be rotated multiple times per day, depending on your traffic log volume. For example, they automatically rotate when the traffic log hits 2 GB. I have one environment which gets over 40 GB of log data per day, so the files rotate a lot.
After traffic logs, the next big items are core dumps as Tal mentioned, then CPUSE packages (check 'installer delete' in clish to see what packages you can delete. Backups saved to /var/log/CPbackup and snapshots exported to /var/log/CPsnapshot also take up a lot of space. Note that snapshots you don't export don't take up any space in the filesystem (they are stored in unallocated space in the drive).
