I know I've got a similar pattern in one of my bazillion quilting books.
Not sure of the exact size to cut, but you will basically take a rectangle piece of fabric that is about a 1/2" wider than the widest part of your border that you are going for. If your center block is cut at 5.5", I would probably cut a rectangle about 2.5-3" wide and about 6.5" long. You will cut this diagonally from one corner to the opposite corner. Attach these two pieces to opposite sides of your center block stitching to the bias cut you just made. You will need another rectangle cut the same way for the other two sides. May need to square up your final block to trim away any excess fabric.
Hopefully this makes sense. Been a long day and I'm dog-tired.