If you have a Glowforge Pro and a 12 by 24" plywood board, here’s how you can quickly cut it to size. Create an .svg file with a single horizontal line. Color doesn’t matter. Save it in an easy-to-find place with an obvious name. I called mine " cutter.svg" using a leading space to pop it to the top of the file list and saved it in the directory where I keep all my GF workfiles.
Insert the board perpendicular to the front slot of the Glowforge and align it. Use hold down pins to keep it flat, or see the tip in the next paragraph. Load a project to cut, then click the “Add Artwork” button. Select your cutter.svg file, position it below your project cuts, widen it as necessary and set it to cut. Voila!
To speed the process, measure the height of the GF front slot and cut a length of PVC pipe or a 1x2 board to the height. Use this to prop the hanging-out-the-slot end of the 12x24, keeping it flat against the GF bed.
Before I figured this out, I used a circular saw to cut 4+ inches off the ends of the 12x24 sheet. Dumb. Not only was it overly complicated, but it also left me with an odd bit of wood which was often too narrow for my purposes. Now I can put most of the 12x24 sheet to use.