OS X Mountain Lion: Adjust your display resolution
Display resolution determines the size of text and objects on your screen.
By default, the resolution on your display is set to show the sharpest text and the most detailed images. You can adjust (or scale) the resolution to make text and objects appear larger on your screen, or adjust it so you have more space on your screen. When you scale the resolution to provide more space, text and objects appear smaller.
Some apps may not fit entirely on the screen in some scaled resolutions. Using a scaled resolution may affect performance.
You can connect a second display and adjust its resolution. Once it’s connected, click Scaled and then select a resolution. Press the Option key while you click Scaled to see additional resolutions for the second display.