...but equally useful for the example, which is just about dividing rocks into two groups. We're interested in the process of dividing the rocks, not the criteria for division.
Use 'height' (and maybe blocks instead of rocks, because then it's clearer they stack).
In what way is the word 'size' wrong?
I guess that Lamby means that one should not use "weight" instead of "mass", the two being two very different concepts.
They're very different concepts, but for the sake of the example it doesn't matter which you are measuring, you could equally be measuring the number of atoms in each one if you happen to know it.