XPLMSetWindowGeometryVR
XPLM_API void XPLMSetWindowGeometryVR(
XPLMWindowID inWindowID,
int widthBoxels,
int heightBoxels);
This routine allows you to set the size, in boxels, of a window in VR (that is, a window whose positioning mode is xplm_WindowVR).
Note that you are responsible for ensuring your window is in VR (using XPLMWindowIsInVR()).