Changing the size of the canvas?

[Ulodesk]

I have created a book cover for someone, for Amazon print-on-demand, which measures spine width by paper thickness x pages. I have made an estimate for final page number and sized the back/spine/front cover image accordingly. If the spine width should need to be increased, and thus the entire background layer/canvas, what's the best way to do this? 

I tried enlarging the canvas, which left a transparent sliver on one edge. I tried filling the transparent area with the background layer color but nothing happened. 

So, first, I don't know how to get the added real estate in the background layer, and second. I really would like to avoid having to move each layer over a tiny a mount to recenter.

Thank you!