I think you are asking more than one question, how long and how wide and how many. Rose L gave a good description on how long you should cut them. Don't just cut a long strip,sew it on and cut off the excess because this is how you get wavy borders. As far as how wide they should be, there's no rule of thumb. I would just lay a couple of fabrics out and audition them for both color and width. If you decide to go with two colors, I personally would make the inner one narrower than the outside one. Just personal preference.