Sue, I'm not certain that I am right, but I'd cut a piece about 80 inches long and width of fabric. Fold it end to end so that the selvage is lined up and then fold it in half again. You would wind up with four layers 20 inches long. Place your ruler and cut the width you want - then cut a second piece. (I would trim the selvage off first). I am assuming the top and bottom are shorter, but go ahead and cut those the same length.

You do know that to measure you should fold your quilt top in the middle and measure the middle side to side; then fold it lengthwise and measure the middle top to bottom. When you put your border on you may or may not need to ease in a bit - this is where your walking foot is handy.

Good luck. I often cut four layers with absolutely no problem. Just make sure you don't let your ruler slip on you.