Ctrl+] doesn't work because to do ], i have to click alt + '*' ( == ]), so { ctrl + (alt + '*' == ) ] } increases as if i was just pressing ]. If you are working with the mouse, and not with the stylus, or other drawing pens, using the scroll to increase the brush size is a bit inconvenient as you have to hover to the 'size box' to trigger the resize.
'+' and '-' are universal signs and they are always present singularly in the numpad. They aren't used anywhere in paint.net, why not changing ], that can differ from keyboard to keyboard, to + and -.