The area is 2(1+sqrt(2)) x a^2, approximately 4.8284 times the side squared, so doubling the side quadruples the area.
Across opposite vertices the octagon spans 31.358 in (the long diagonal), which is also the diameter of the circumscribed circle.
The apothem of 14.485 in is the radius of the largest circle that fits inside the shape, and the ratio apothem/circumradius is always about 0.9239 regardless of size.
Interior angles are always 135 degrees and exterior angles are 45 degrees; that is why two octagon roof sections meet a square perfectly at a corner.
Next stepUse the circumradius (15.679 in) to draw the octagon by stepping eight vertices around a circle, or use the apothem to fit the shape snugly inside one.