Backing should be ample. It takes 2-3 inches to to get in onto the rollers top and bottom, and frequently tops are not sewn perfectly square, or not loaded onto the frame perfectly straight, so some wiggle room on the sides is also needed. Make your backing 8 inches longer and wider than the widest/longest measurement of your top. If your back is pieced and has a seam, your longarmer will load it so the seam rolls up parallel to the roller bars to avoid one section getting thicker as the quilt is taken up on the bar. If you have a directional preference, be sure to label that for your quilter.
