I don't think of a piano key border as having any restrictions on width or length. Just pick a size, and sew the strips together. I've used that type of border finishing at 1" x 5" or 6" on log cabin quilts, and recently made what I called a chunky piano key border using strips that were several inches wide. What is the problem you're having? Are you uncertain how to do the corners, or perhaps how wide to make the "keys"?