Divide the number of stitches vertically and horizontally. The result will be the stitching area in inches. Then add six inches vertical and six inches horizontal
For example: Assume the stitching area is 81 x 90
81 divided by 9 is 9
90 divides by 9 is 10
The new stitching area is now 9 inches x 10 inches
Add six inches each way. that comes to 15 inches x 16 inches. That is the amount of fabric you need. If you are using 9 ct fabric you will also need a thicker thread. Embroidery floss probably would not cover well.