VMS Help MERGE, Input File Qualifier, /FORMAT, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Sort obtains the file's longest record length (LRL) and file size
    from RMS. If you know the LRL that RMS has defined for the input
    files is incorrect, you can override this value by specifying the
    record size with RECORD_SIZE. For multiple input files, LRL is
    the length of the longest record in all files.

    If you do not know the LRL value for a file, use the ANALYZE/RMS_
    FILE command. The LRL value appears in the file attributes
    section in the statistical report generated for the file that
    you specify.

    Sort uses input file size information to determine the amount
    of memory needed, as well as the size of the work files for the
    sort operation. If the file size is unknown (for example, you
    are sorting files not residing on disk or standard ANSI magnetic
    tape), Sort assumes a fairly large file size.

    If this default is too large, Sort overestimates its memory and
    work file requirements; the sort operation will be more efficient
    if you specify a smaller input file size. If the default is too
    small, Sort underestimates its memory requirements; therefore,
    you should specify a larger input file size, provided the Sort
    data structure size is not limited by the working set extent.