If you're using the iPad version you can change orientation by going to the Gallery, scrolling to your image, and using the rotate buttons on the toolbar.
If you're using the desktop version of ArtRage, you can change the size of the canvas using the Crop/Expand the Canvas option in the Edit menu. In the dialog that appears, turn off 'Preserve Original Aspect' and switch the width and height values displayed. At the bottom of the screen you'll see a preview of where the current contents of the canvas will be placed when you apply the change, so if you have something in there already you can position it within the new area.
Matt
ArtRage UI
Ambient Design.