In CS4 there's a new option that allows you to control the brush tool attributes, size and hardness, using the mouse to show you how it'll look. When you have a brush tool open hold down the "Control" and "Option" keys on the keyboard (these are the keys on a Mac, not sure about a Windows computer) and then move your mouse. As you move the mouse, it'll increase and decrease the size of the brush and show you the result directly on the screen. You can then hold down the "Command" key in addition to the other two keys and as you move the mouse it'll control the hardness of the brush and show you the result directly on the screen.