> Document Settings
> Typically the canvas size is fixed to the initial screen size when you begin capturing. Document Settings enables you to change the canvas size the document you have created.
> To do this select File > Document Settings.
> When the canvas settings window displays, select a preset and enter the width and height needed. Click Update to immediately resize the canvas to the new dimensions.
> .
> These settings are very similar to the options offered when creating a new document.