Function XPLMSetWindowGeometryVR 

pub unsafe extern "C" fn XPLMSetWindowGeometryVR(
    inWindowID: XPLMWindowID,
    widthBoxels: c_int,
    heightBoxels: c_int,
)

XPLMSetWindowGeometryVR

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()).