Fortisiem can receive Syslog event messages of various sizes, but all Appliances are configured with a default maximum event size.
Messages that are larger than the maximum size of the RFC specification for the TCP and UDP protocol have their event payloads truncated to ensure that Fortisiem can receive the event. Maximum event size by protocol: UDP syslog messages should not exceed 1024 bytes TCP syslog messages should not exceed 8192 bytes.
There is no minimum length for the syslog message. Overall performance would be affected if the syslog size limit is increased, more data sent through, more resources needed to process.
Unfortunately, there is no settings on Fortisiem that can allow to increase size of the packet. If you want to increase the size limit, you need to raise a feature request.