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

       /WIDTH=characters-per-line

    Specifies the maximum characters per line. This value must be
    an integer in the range 1 to 511. With the /WRAP qualifier, the
    terminal generates a carriage return and line feed when the width
    specification is reached.

    If the specified width on an ANSI terminal is 132, the screen is
    set to 132-character mode. If the terminal does not have advanced
    video option (AVO), the page length limit is set to 14 lines.