VMS Help PHONE, Qualifiers, /VIEWPORT_SIZE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /VIEWPORT_SIZE=n

    Specifies the maximum number of lines in a viewport, including
    the heading line and the bottom line of dashes. The valid range
    is 3 to 10 lines. Smaller viewports will improve the response
    time on terminals operating at slow speeds. The default viewport
    size is automatically reduced as more participants enter the
    conversation.