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

       /EXTENSION[=n]

    Specifies the number of blocks to be used as a default extension
    size for all files on the volume. You can specify a value n in
    the range of 0 to 65,535. If you specify the /EXTENSION qualifier
    without specifying a value, a default value of 0 (the OpenVMS RMS
    default) is used.

    For example, during an update operation, the extension default
    is used when a file increases to a size greater than its initial
    default allocation.