/sys$common/syshlp/helplib.hlb SET, HOST, Qualifiers, /BUFFER_SIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /BUFFER_SIZE=n

    Changes the packet size of the protocol message sent between the
    terminal and the remote processor if a connection to the remote
    processor is already established. The default buffer size is 1010
    bytes; however, the buffer size n can range from 140 bytes to
    1024 bytes. The value of the parameter n is reset to 140 bytes if
    a value below 140 is specified; a value for n above 1024 bytes is
    reset to 1024.

    You can force the host node to write to the terminal in smaller
    packets, thereby ensuring that write operations to the terminal
    are displayed at more frequent intervals, by setting n to a
    value just above the minimum of 140 bytes. On slow DECnet links,
    setting the buffer size to a smaller value may decrease pauses
    between write operations when large amounts of data are being
    scrolled to the screen.