VMS Help Sys Parameters, MAXBUF  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    MAXBUF sets the maximum allowable size for any single buffered
    I/O packet. Buffered I/O packets are allocated from the
    permanently resident nonpaged dynamic pool. The terminal,
    mailbox, and printer device drivers are examples of device
    drivers that perform buffered I/O.

    The number of bytes specified in the I/O request plus the size of
    a driver-dependent and function-dependent header area determine
    the required buffered I/O packet size. The size of the header
    area is a minimum of 16 bytes; there is no absolute upper limit.
    However, this header area is usually a few hundred bytes in size.

    On OpenVMS VAX systems beginning with Version 7.1, the default
    value is 4112. The default value on OpenVMS Alpha systems
    continues to be 8192.

    The maximum value of MAXBUF is 64000 bytes.

    MAXBUF is a DYNAMIC parameter.