There’s a Beta button on the side of the controller that you draw with. When you click it, you can edit the proportions of the current brush, whether built-in or one you’ve copied/loaded.
I think it’s the only time you “point” with the controller that has the menu on it. And you point it at the controller that does the drawing. Not at a menu like everything else. (Or, it is… but it’s just stuck on the side of the controller, so I thought it was just a label.) So it took me forever to find this one!
Edit: Yes, I know Jakers said basically this, but thought I’d interject because of my earlier confusion.