Short help:
Log type:
Whether your log will be stored in a memory circular buffer or in a file. Beware that files are stored in a memory filesystem which will be lost if you reboot your router.
Log File:
The path and name of your log file. It can be set on any writable filesystem. CAUTION: DO NOT USE A JFFS filesystem because syslog will write A LOT to it. You can use /tmp or any filesystem on an external storage unit.
Log Size:
The size of your log in kibibytes. Be carefull with the size of the circular buffer as it is taken from your main memory.