This is another feature that is related to antivirus scanning. The FortiGate unit has a finite amount of resources that can be used to buffer and scan a file. If a large file such as an ISO image or video file was to be downloaded this could not only overwhelm the memory of the FortiGate, especially if there were other large files being downloaded at the same time, but could exceed it as well. For this reason, how to treat large files needs to be addressed.

A threshold is assigned to determine what should be considered an oversize file or email. This can be set at any size from 1 MB to 50 MB. Any file or email over this threshold will not be processed by the Antivirus Security Profiles. Once a file is determined to be oversized it must be then determined whether to allow it or to block it.

These settings are not a technical decision but a policy one that will depend on your comfort level with letting files into your network. As there often is, there is a compromise between convenience or ease of use and security. If you want to go for a high peace of mind level you can configure the firewall to block oversized files and thus no files would be coming into the network that have not been scanned. If you are looking for optimizing the memory of the FortiGate unit and making sure that everybody is getting the files they want, you can lower the threshold and allow files that are over the threshold.