in this tutorial i will explain how you can change the brush size to a larger or smaller size than the 3 preset sizes, most people think that they only have 3 choices of brush sizes in Ms Paint, this is not true. you are in full control of your brush.
Using the brush tool select the smallest round brush tool and stamp it on the canvas
then the middle round brush tool and stamp on the canvas and finaly the largest round brush and stamp on the canvas, now to get a larger brush size simply hold down the Ctrl button and hit the plus button on you numeric keypad this will increase the size of your brush hit the minus button to decrease hit a preset button to resize the brush all together. see fig.1 below.
the same technique also applies to the line, Spraypaint and the Eraser tool.
so thats how you can control your brush sizes in Microsoft paint.