Select a device implementing ACCOUNTING-CONTROL-MIB as the target host to create a monitor — the SNMP service should be up and running on it. Click New Monitor, then check SNMP Custom on the Favorites tab, click Next, and confirm the host. On the next page, click Select... to open the built-in SNMP MIB Browser and locate acctngFileCurrentSize under the acctngFileEntry branch of ACCOUNTING-CONTROL-MIB, selecting the specific row/instance you want to monitor since this is a table column, then select it and click OK. It reports, in bytes, the current size of the accounting file currently being written to. On the monitor's Main parameters page you can set the target's SNMP port (default 161), credentials, polling interval, and other settings — see the SNMP Monitor help for details. On the State conditions and Alerting tabs, configure when the monitor should change state and trigger an alert; since this is an Integer32-type OID, Value bounds is the most useful condition here — trigger an alert if the file size exceeds a size close to the configured accounting-file limit (e.g. 90% of the configured maximum), so the file can be rotated before data is lost. Click Finish to create the monitor; you can adjust any parameter later.