VMS Help MOUNT, Qualifiers, /EXTENSION  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /EXTENSION=n

    Specifies the number of blocks by which disk files are to
    be extended on the volume unless otherwise specified by an
    individual command or program request.

    The parameter, n, specifies a value from 0 to 65,535 to override
    the value specified when the volume was initialized.

    Example

    The following command mounts a volume labeled DOC on DKA0,
    assigns the logical name WORK, and specifies a default block
    extent of 64 for the files on WORK:

    $ MOUNT/EXTENSION=64 DKA0: DOC WORK