Sorry can't find this simple feature. I was asked for document size at creation time but don't know how to change this later.
Select the Artboard Tool on the Tool bar.
You can then click an artboard and change it's size with the options in the Control bar across the top of the screen.
Another method is to highlight the artboard in the Artboard Panel (Window > Artboards) and choose Artboard Options from the Panel menu.