XPLM_API void XPLMSetWindowGeometry( XPLMWindowID inWindowID, int inLeft, int inTop, int inRight, int inBottom);
This routine allows you to set the position and size of a window.
The units and coordinate system match those of XPLMGetWindowGeometry(). That is, modern windows use global desktop boxel coordinates, while legacy windows use pixels relative to the main X-Plane display.
Note that this only applies to “floating” windows (that is, windows that are drawn within the X-Plane simulation windows, rather than being “popped out” into their own first-class operating system windows). To set the position of windows whose positioning mode is xplm_WindowPopOut, you’ll need to instead use XPLMSetWindowGeometryOS().