15″ wide by 72″ long worked perfectly for my table. The best way to get a true straight line in any material is to pull a thread–fortunately, the coarse nature of burlap makes that very easy. In this case, a thread was pulled the 72″ length of the burlap to remove the selvage (the finished edge). The selvege is a tighter weave and needs to be taken off to allow for fringing in the final step.
After the thread has been pulled, use the empty space left in the weave as a guideline to cut the burlap. This technique works in both directions and was used to cut not only the runner but the placemats, too.
Be forwarned that burlap can create a great deal of dust. Best solution is to use the great outdoors, the basement or garage in order to make clean-up simple. In my case, my best laid plans were temporarily delayed because of my helper. Barbie LaMew decided that the cardboard box that had been set up to capture the discarded threads was the perfect spot for her afternoon nap. Sigh.