I agree with JJean except I lay the quilt on top of the border fabric to audition border widths. You can play with the width without cutting into the fabric. I try to have the border relate to something in the quilt, especially for pieced blocks.
Also allow a little extra in your quilt size if you do not preshrink your batting. I use bamboo/organic cotton batting for kids quilts. bamboo is supposed to be bacteriostatic.
Jeanne