get the size of viewable screen area?

How do i get the width and height size of the viewable screen area?

The viewport expressions work fine until i change the scale size of the layer from 1.0, zooming out works correctly like 0.5 but zooming in causes the trouble like 2.0 seems to return a much bigger value even though it's still less then the size at 1.0.

I have integer letterbox scaling enabled.

Thanks.