How do you access the a canvas's width and height when canvas' render mode is screen space camera?

The title says it all. I’d like to know the width and height of the canvas in world space, not screen space. I know that when the canvas’ render mode is in screen space overlay, the width and height are the same as Screen.width and Screen.height. Any help is appreciated.

float panelWidth = menuCanvasRectTransform.rect.width * menuCanvasRectTransform.localScale.x;
float panelHeight = menuCanvasRectTransform.rect.height * menuCanvasRectTransform.localScale.y;

void Start()
{
RectTransform parentCanvas = GetComponentInParent().GetComponent();
Vector2 canvasWidthHeight = new Vector2(parentCanvas.rect.width, parentCanvas.rect.height);
}
And it’s important that this is inside Start() and not Awake().