Function XPLMSetWindowGeometryOS 

pub unsafe extern "C" fn XPLMSetWindowGeometryOS(
    inWindowID: XPLMWindowID,
    inLeft: c_int,
    inTop: c_int,
    inRight: c_int,
    inBottom: c_int,
)

XPLMSetWindowGeometryOS

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()).