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

       /EXTENSION=number-of-blocks

    Specifies, for disk volumes, the number of blocks to use as a
    default extension size for all files on the volume. The extension
    default is used when a file increases to a size greater than its
    initial default allocation during an update. For Files-11 On-
    Disk Structure Level 2 disks, the value for the number-of-blocks
    parameter can range from 0 to 65,535. The default value is 5.
    For Files-11 On-Disk Structure Level 1 disks, the value can range
    from 0 to 255.

    The OpenVMS operating system uses the default volume extension
    only if no different extension has been set for the file and no
    default extension has been set for the process by using the SET
    RMS_DEFAULT command.