https://datatracker.ietf.org/doc/html/rfc6985
...
iperf3 -c hostname -T s1 -p 5101 &;
iperf3 -c hostname -T s2 -p 5102 &;
iperf3 -c hostname -T s3 -p 5103 &;
Some >15 y ago people measured the distributions on a real network and came up with IMIX409 with an AVERAGE packet size of 409 bytes.