How big is it?
You can make this block any size you would like. All you need to do is to assign a size to each patch of the block. Imagine a 3x3 grid over the block. If you assign 2" (finished) to each patch, the block would be 6x6" finished. If you assign 1" (finished) to each patch, the block would be 3x3" finished.
Your border should be in scale with your quilt ... if your quilt center is delicate, you wouldn't want an overwhelmingly big block in the border. Unless you are planning your quilt in advance on the computer or on graph paper, you might want to wait and "audition" a border when your quilt center is done.
How do I create it?
Quilters use a 1/4" seam allowance. Therefore you will need to add 1/2" to the finished measurement when cutting. Let's say each unit of our border block will be 2" finished ... the block will be 6x6" finished. We'll start by cutting a 2-1/2" strip of magenta and a 4-1/2" strip of black. Seam them together lengthwise, then cut crosswise into 2-1/2" x 6-1/2" units. Next cut 2-1/2" strips each of teal, magenta and black. Seam together lengthwise, and cut crosswise into 2-1/2" x 6-1/2" units. Finally, cut a 4-1/2" strip fo teal and a 2-1/2" strip of magenta, and cut crosswise into 2-1/2" x 6-1/2" units.
Stitch your rows together to form the 9-Patch Border block:
Because of the way the 9-Patch block is colored, you get a nice wavy border around your quilt. You can use either a plain block or a pieced block to turn the corners.
Related : More Free Border Patterns