In my experience, it is generally not a good idea to change the scale of individual objects in the UI. If you want to change the size, it is better to work with the size in pixels.

If you want the size of the UI to adapt to different resolutions, you should use a Canvas Scaler, which automatically adjusts the scale of a canvas., Within this canvas you should not change the scale.

If there is really no other way, you will have to get out a calculator. If the child has a scale of 3 and the parent has a scale of 1, the difference (top and right variable) must be the size of the canvas times 2/3 (I think).