The max-udp-size option sets the
maximum EDNS UDP message size named will send. Valid
values are 512 to 4096 bytes (values outside this range will
be silently adjusted). This option is useful when you
know that there is a firewall that is blocking large
replies from named.
It is independent of the advertised receive buffer (edns-udp-size).
Sorry it is not in your version. It was introduced in 9.4.0.