> setting the size of pencil is completely unintuitive - I usually click in what appears to be input field, only to discover it is not (but instead some meter that sets size to some huge number)

It actually is an input field, but it can be interacted with in 3 ways. Clicking on the value focuses it, and allows editing by keyboard. Clicking and dragging in the bottom half of the block allows you to increase or decrease the value by dragging left or right, and the movements you make translate to relatively smaller value changes, so you can be relatively precise. Clicking and dragging in the top half of the block allows you to select a specific value by dragging, and the position of the cursor translates directly to the position of that value on the bar.