If you decide to use 14 ct fabric the stitched size would be 11.5 x 20.5 inches. You should buy a piece of fabric that is 18 x 27 inches.
What do you mean by change your opinion? You can always make any changes that you want in colors or fabrics. The designer makes suggestions. The cross stitch police won't come and take you away. Personalize your projects in any way you want.