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

       /ALLOCATION=number-of-blocks

    Forces the initial allocation of the output file to the
    specified number of 512-byte blocks. If you do not specify the
    /ALLOCATION qualifier, or if you specify it without the number-
    of-blocks parameter, the initial allocation of the output file is
    determined by the size of the input file.

    The allocation size is applied only if a new file is actually
    created by using the /NEW_VERSION qualifier.