How to Change the Artboard Size in Adobe Illustrator
The artboard in Adobe Illustrator is the printable area around an image. It's roughly analogous to the canvas from other graphics editors, but Illustrator has its own canvas that the artboard is superimposed on. The artboard optimizes your working area for printed output rather than general editing, and a single Illustrator document can contain up to 100 artboards. You can either freely re-size artboards by clicking and dragging, or you can specify the exact dimensions you want.
Freely Resize Artboards
Click the artboard tool from the toolbar.
Hover the mouse over a corner of an artboard. The cursor will change to a double-sided arrow.
Click and drag to change the artboard's size.
Specify Precise Dimensions
Double-click the artboard tool from the toolbar. This opens the "Artboard Options" dialog box.
Type the width that you want, measured in pixels, in the "Width" text box.
Type the height that you want, measured in pixels, in the "Height" text box.