Changing the strip width when cutting the strips prior to sewing the stip sets will make a quilt longer. Changing the width of the cutting of the sewn strip sets makes the quilt wider. For every 1/4 inch you add you gain 1/2 inch , so count the seams and multiply by .5 . that is the total number of inches. This works in reverse as well.
If you want to increase/or decrease the total size both dimensions , both the unsewn strips cutting and the sewn strip sets needs to be modified .