XPLM_API void       XPLMSetWindowGeometryOS(
                         XPLMWindowID         inWindowID,    
                         int                  inLeft,    
                         int                  inTop,    
                         int                  inRight,    
                         int                  inBottom);    

This routine allows you to set the position and size, in operating system pixel coordinates, of a popped out window (that is, a window whose positioning mode is xplm_WindowPopOut, which exists outside the X-Plane simulation window, in its own first-class operating system window).

Note that you are responsible for ensuring both that your window is popped out (using XPLMWindowIsPoppedOut()) and that a monitor really exists at the OS coordinates you provide (using XPLMGetAllMonitorBoundsOS()).