The angle for the octagon is the same no matter what length the outer side is. So if you want to cut 16 inch boards (presumably so you can get exactly 6 pieces from an 8 foot 2 by 4), you still want to cut angles of 67.5 degrees. A little more trigonometry shows that the diameter of the inner octagon will be 34.2 inches for 16 inch outer sides. In terms of area, the area of any octagon with radius r is . Applying this you get for 16 inch outer sides, an area of 5.75 square feet; for your original question, the inner area is 6.36 square feet.