Step 2: Make a paper stencil/guide
I ended up with the front and the back (the rectangular pieces) being 11" x 13". I also rounded the bottom of this piece for purely aesthetic purposes.
The long strip that connected the front to the back (and makes up the bottom and sides of the bag) was 3" x 36" (I cut it extra long; better too long than too short).
When I was satisfied with the dimensions, I cut the model apart so I could use them as a pattern on the cardboard.