Changing the size of the RectTransform's rect

I was trying to copy another UI's rect from it's RectTransform in a different parent, unfortunately the rect property is read-only. I tried sizeDelta but it's relative to it's parents and it doesn't really work for me.

And here's the result when trying to use SizeDelta - the code is

sceneScaler.sizeDelta = viewportManager.sizeDelta

Is there anyway for me to be able to do this? [I need both of them to be the same size]