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

       /EXTENSION[=n]

    Sets the extend quantity default for the file. The value of
    the parameter n can range from 0 to 65,535. If you omit the
    value specification or specify a value of 0, OpenVMS Record
    Management Services (OpenVMS RMS) calculates its own value for
    the /EXTENSION qualifier.

    See the SET RMS_DEFAULT command for a description of the /EXTEND_
    QUANTITY qualifier.