Open a printer emulation session.
From the Setup menu, choose View Settings.
This setting specifies the size of the margin to be left at the right side of a page of printed output. The units of measure are twips. (A twip equals one twentieth of a point. There are 1440 twips to an inch.)
As you change your left or right margin, the value for Printer Chars per Line may change as well — larger margins mean fewer characters per line.