PR1355087 has introduced "snmp packet-size" to let customer manually set the SNMP packets size to reduce the path MTU effects. But this knob doesn't work for SNMPv3.
Even if [snmp packet-size <size>] is configured, the responded SNMPv3 packet could be of larger size.
PR1817865 has fixed this problem for both JUNOS and EVO version.Another way to avoid such problem is to send snmpv3 request from the NMS with msgMaxSize value less than the configured 'snmp packet-size' value.