This command configures the layout of the Single Logical screen feature. It takes two arguments, Horizontal and Vertical which must be an integer value dividing evenly into the total desktop width, and height. For an example with two monitors side by side which appear as one screen through the X-Server with the right screen as the primary screen, use:

XineramaSlsSize 2x1
XineramaSls On
XineramaPrimaryScreen 1
Xinerama On