How to use Canvas Scaler's "Constant Physical Size" for buttons?

I’m a little confused on how to measure the constant physical size when I’m positioning/scaling a button in the Unity Editor in a Canvas whose UI Scale Mode is set to “Constant Physical Size”. I want the button to be a fixed 3 cm x 3 cm.

So I’ve set the Physical Unit to “Centimeters”, but then I don’t know how many cm the button is. There’s no indicator of the proportions as I’m resizing it, I only have the Game/Scene view for reference. However the Game/Scene view isn’t measured in cm nor has any reference to an actual size in cm (only pixels, which converts differently to cm based on ppi), so I’m not sure what to do.

Do I need to code this as opposed to doing it directly in the Unity Editor?

You can do it, first you need to know how big your canvas is, you can do this setting a cube near it.
Secondly you have to know that instead of scaling is better to change the anchors.
The 100 (if you have 100 as the pixel size) will be the canvas scale, the next is a rule of 3 like if 100 = 2m
3cm will be 0.03/2*100