XPLMSetWindowGeometryOS
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()).