The nicer hotels have been using these for years over a white duvet. They're classy. I have always called them scarves rather than runners, but either way, they dress up a bed without all the work that goes into a full size quilt.
They are about 2 ft. by the width of the bed with a drop of about 18 inches on each side. In other words.........measure the width of the bed and add 3 ft.