This article explains how to customize your Mac's Dock, changing both the appearance and position of the dock itself, as well as which icons appear in it. Instructions apply to Macs running OS X 10.7 (Lion) and later.

Open System Preferences from the Apple menu.

The Size slider determines how large icons appear in the Dock. As you move it, the Dock will appear so you can preview your changes.

Click the checkbox next to Magnification to enable this setting. When Magnification is enabled, app icons will become larger when you hover your mouse over them, making them easier to see.