VMS Help DECnet-Plus, DCL Commands, SET, HOST, /X29, Qualifiers, /PACKET_SIZE=number  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /NOPACKET_SIZE

    (Default=/NOPACKET_SIZE)

    Use this qualifier to request the  specified  packetsize  in  the
    call  packet.   Otherwise,  the  network  default  is  used.  The
    packetsize should be between 16 and 1024 and must be a  power  of
    2.   If  you request a packetsize larger than the one configured,
    X.25 will use the largest size configured.