I agree with the idea that no matter the starting size of the squares, the "loss" will be the "same" no matter what.
To make it easy on yourself, cut 9 squares .
sew them together, and then measure what you have
Then cut the 9-patch unit apart,
sew the four pieces together, and then measure what you have.
Then remember to subtract 1/2 inch for seam allowances for the "finished size" of the unit.
Without actually going downstairs and checking it out for myself, my guess is that the finished size unit will be two inches "lost" in seam allowances.
Let's try that with nine 5 inch squares.
After sewing them together into the basic 9-patch, it measures 14 x 14 inches. (13.5 x 13.5 inches "finished" size)
After cutting in apart, and sewing the four units together, it measures 13.5 inches (13 inches "finished" size)
Try it with some scrap fabric and see how it turns out.