Library /sys$common/syshlp/dtsdtr.hlb DATA, COMMAND-SPECIFIC QUALIFIERS, /SIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /SIZE=number

    Indicates the data message length in bytes. The lower bounds are 0 for
    a sink or echo test; 4 for a sequence test; and 5 for a pattern test.
    The upper bound is 4096 bytes for all tests. The default size is 128.