I always forget to write down the paper size for covering composition books. I give these away these for gifts all the time with a matching pen.
It is 8 1/2" x 11". I fold it over the cover after spraying on the adhesive.
I found couple of videos on Youtube.com that did it differently. A lady did one by cutting the 12" x 12" in half and adding a strip to cover the remaining section. Another lady actually painted the notebook after sanding the composition book down inside and outside. I like both ideas and will be trying them out. :-)