Additional way how this could work with current systems is:
Atm, i use S and D keys to increase or decrease for the brush size. If my current brush size is set to 3px and i want to go from 3px to 50px i have to either push the D keys x times until i hit 50px or, i can hold D and with slight delay it starts to increase auto smoothly and hit 50px. When Painting, im using Painting Mode with the color, saturation, value sliders, and the only way of changing brush size comes from these keys, S & D.
The Additional Way: While holding the S or D, additional 3rd function event could trigger if pen input along brush size key input, that would allow to change the brush size the way its shown above in the first post. While holding S or D and sliding the Pen on screen, you could adjust smoothly the size at the hit point location of the pen and screen, adjust size and release S or D and paint with adjusted size.
This additional size adjusting would help with custom preset experience listed below:
I also use 5 custom presets of different Brush Types assigned to key shortcuts constantly. These have their own brush sizes, when switching from one preset to the other, the value will change to the root value of the preset it was created with. Switching back and forth between two or more custom presets, i find myself having alot to adjust the brush size of one or the other back to the size value i was painting with before a switch. If for example i painted with preset1, that is hard round brush with root size 10px and i increase it with D-key to 15px and hit shortcut shift+D that is assigned to preset1 i am painting with, the size of my brush resets back to 10px, that is the root size of the custom preset1. If i use preset preset1 and increase the brush size with D-key to 15px and switch to different preset2, the size value will change to the root size of the preset2. The more of custom brush presets i use, the more i find myself to adjust the size value after preset switch. The more excess moves i have, to adjust the brush size, the more time, energy and pleasure of user experience is consumed for the sake of high effective focused painting mode.
Solution: Add enabling option that allows to transfer current brush size value between different brush presets. If my current size is 3px, switching between different presents wont change the 3px.