Measure across the center the quilt top legthwise and cut it from that measurement not what is showing on the actual sides. Do the same for the width ... through the center of the quilt. If the borders have a direction you must cut the long wise for the long borders and the width for the width of the quilt. Then all of the prints are traveling the proper direction. You see? Fold it in half and pin in the center and the ends then work out from there with your border strips.
It is not a bad idea to do it this way all the time, even if there isn't a direction for the print on the fabric. You see, the long wise has more threads running through it. These are called warp and weft of the fabric. The reason is the strength of the fabric is on the edge of the finished top and will not wave around after the borders are on as easily when you are finished quilting. Use the strength of the fabric to help stablize the edges.
As far as the width goes, I like to use whatever looks the best with my design. Usually it runs five inches finished if I am using a seven inch blocks. But, sometimes just about one third of the block size you were working with in the top looks good.
I don't quilt for shows or have any real instruction, per say. But, I know what I like and I do that. So, what I am trying to say is. I am not a teacher. You might hear something else from someone who really knows the "correct" width to use.