Hello,
This design is a Heaven and Earth design and their designs are stitched on 25ct. White linen over one thread (instead of 2 like it's usually done on linen) So with the stitch count of 400 x 319 over 1 thread on 25ct. linen, leaving 6 inches on each side, you will need a piece of fabric size of 28" x 25" and your stitched area will be 16" x 12 3/4". Here is the link to the calculator I used to get these dimensions...I use this one because of all of the options it lets you choose from....http://www.crosstitch.com/calc.html
Hope this helps!!
~Kristi @ ECS