Library /sys$common/syshlp/nclhelp.hlb set, x25_protocol, dte, pvc, packet_size  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: No default    Value: 16-4096

    Packet size for the PVC, in octets. The value must be a power
    of 2 in the range 16 to 4096. The value of this characteristic
    derives from an argument to the create command.

    Example:

    set x25 protocol dte dte-1 pvc PVC-1 packet size 128