The 5" was suggested in the other post because she was trying to get to a specific finished quilt size. As Dunster said, it needs to be proportional to the quilt and the blocks. As far as the corners go, I've done it different ways. You can use a plain or pieced block in the corner or apply it just like a normal border (sides, then top). In order to miter it with seams matching, you have to be exact in the calculation of the width of the keys so that the border exactly fits the quilt top.