Measurements of my FINISHED King is 20X34 (you will need at least 40 X 34 inches of fabric) from end to beginning of the cuff. You will need to add your seam allowance. If you are doing french seams add between 3/4 to 1 inch if serging add at least 1/2 inch to the to both measurements.
Does this give you an estimate?
Was posting the same time as crafty...