Should table runners be as long as round table cloths?

For my October Wedding, I would like to add a little something more to the round,plain white reception table linens. I ordered sheer sparkly chocolate colored fabric that i would like to make into table runners. I will be using 2 runners per table. They will cross at the center where the centerpieces will sit. Should the runners be the same length as the cloths? The cloths hang 6 inches over the table edge. Thanks so much for any info.