First, an upright and a grand are simply not the same instrument. Gravity-based action alone should suffice to differentiate these cousins. No one wold consider a clavicytherium a harpsichord, even though both are plucked string keyboard instruments. Same difference with the upright and the grand. 88-keyed hammer keyboard instruments, yes, but not comparable.
That said, you can have a very good upright and a very bad grand, and the former will be more pleasant to play than the latter. Still, different instruments that cannot be compared on equal footing.
Then, grands and size: you can't teach tall. The shorter the scale, the thicker and tubbier the bass strings. A thick string does not vibrate very nicely, so you get that godawful sound that I think of when I remember grands smaller than 6'10". Yuck. The longer scales have thin bass strings (the thinner, the better), resulting in a cleaner sound.
But there is so much more to piano design than the speaking length of the lower strings! Soundboard design, ribbing, rim bracing, point at which strings cross, bridge placement, and a myriad of other factors have a decisive influence on the acoustic properties of pianos, and the whole question of the action, with the balance, inertia, mass, hardness, and weight of hammer and keys, and the excruciating nuance of friction inherent in all those little pieces between the key and the hammer, very immediately will make the pianist fall in love with the instrument of hate it.
You of course can have a piano with a very good action, which feels good to play, but sounds not so good because the scale is too short (I am thinking of one of those exquisite 190 cm instruments made by high-end makers), and some other instruments that are horrible to play, even though they have the size to sound better (like those low-end pianos in their larger models).
You can fix an action much better than fix a scale, though, so when in doubt, get the bigger one and then work on the action. You can teach a kid to shoot the ball better, but you can't teach tall.
The myths are not busted. They are generally true.