code: func (m *Monitor) PhysicalSize() (width, height float64) {
  	var wi, hi int
  	mainthread.Call(func() {
  		wi, hi = m.monitor.GetPhysicalSize()
  	})
  	width = float64(wi)
  	height = float64(hi)
  	return
  }
docs: // PhysicalSize returns the size of the display area of the Monitor in millimeters.