Yes, it is mostly for Character Customization. I thought of anything between 0.20m (almost 1 block tall) and maybe 9.95m (almost 40 blocks tall) as valid choices in a vanilla Game. Though if someone were to make a Mech Suit Mod, it would likely use the same Mechanics as just a very tall person would.
As for speed selection, I thought about a speed-up and a speed-down button, in which case I would also put sneaking (or maybe even crawling flat on the ground) as the slowest speed instead of the uniform walking. That way you would need to “shift gear” from slow to fast before running away when you get caught stealing or something.
Though you could just bind a specific speed to a special key if you like to, which is a good way of showing others that you are a professional thief if you can go from sneaking to running with just one keypress. XD