How we built Flow
Not certain their use case of brush size has any relevance. Most graphics designers have a tablet and can easily set brush size to pressure control, or they just hold down the hot key that lets you adjust brush size by moving your mouse/pen so they already have analog input to brush size any time they want. Maybe some of the other professions they mention are more lacking in equipment, however. It is kind of weird talking about how hard it is to get to the brush size dialog, though, when no one uses that unless they are an amateur...