Customizing the Dock
After you've used the Dock for a while, you'll probably want to customize it to better suit your needs.
If you have a small monitor, you might want to resize the Dock icons to cover less area. The easiest and fastest way to resize them is to click and hold on the separator bar that divides the Dock areas. As you click and hold on the separator bar, drag up and down or left and right (if your Dock is placed vertically). The Dock dynamically resizes as you move your mouse. Let go of the mouse button when the Dock reaches the size you want.
After playing with different Dock sizes, you might notice that some sizes look better than others. That's because Mac OS X icons come in several native icon sizes, and points between those sizes are scaled images. To choose only native icon sizes, hold down the Option key while using the separator bar to resize.