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

       /NETWORK_BLOCK_COUNT=count

    Specifies a default block count (0 to 127) for network access to
    remote files, where count is the number of blocks to be allocated
    for each I/O buffer.

    For remote file access, the buffer size is negotiated between
    RMS and the remote system's file access listener (FAL) with the
    smaller of the two sizes being selected.

    Thus, the /NETWORK_BLOCK_COUNT value places an upper limit on
    the network buffer size that is used. It also places an upper
    limit on the largest record that may be transferred to or from
    a remote file. The largest record must be less than or equal to
    512*network_block_count. (The network_block_count is in block
    units and the record is in byte units.)

    If you omit the value or specify a value of 0, RMS uses the
    systemwide block count value. If this value is also 0, RMS uses a
    size of one block.