Preview Width is a feature that is exclusive to the responsive design canvas. You will be able to select how the view is displayed across devices. You will also be able to build the page while in the preview width model, to show what it will look like while building for a specifically sized device.
The Preview Width tab
1600px: This is for desktop computers and most laptops. This is what you are started in when the responsive canvas loads up.
1000px: This is for some laptops and tablet devices such as the iPad.
800px: This is for mobile devices that are in landscape mode.
600px: This is for mobile devices in portrait mode.