The sMaxPhysWidth box contains a value which specifies the maximum allowable physical page width in master units.
For the Diconix 150 Plus printer, the maximum physical page width the printer can handle is 8.5 inches. Converting this value to horizontal master-units (using the Xdpi value from the master-units dialog box), we obtain the value 8160. This is the value that appears in the sMaxPhysWidth control.
For the HP LaserJet IIP printer, the maximum printable page width is 8.5 inches. Converting this value to horizontal master units (300 * 8.5 = 2550), we obtain the value entered in the sMaxPhysWidth box.