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

       /EXTENT=n

    Specifies the maximum number of physical pages that can be
    resident in the working set during image execution.

    The extent value must be greater than the minimum working set
    defined at system generation, and it must be less than or equal
    to the authorized extent defined in the UAF.

    If you specify a value greater than the authorized extent, the
    command sets the working set limit at the maximum authorized
    value.