OK - I figured out the mfg's wing area measurement.

Using a fabric tape, I measured the circumference on the wing at the tip and root. On the Extra, that gives 23 3/8 at the tip, 42 3/8 at the root, and a length of 39 5/8.

This gives an area of 1301 sq in. on the wings. The fuselage is 6 1/2 inches wide, but flat, so this adds 128 1/2 giving exactly 1429 1/2 sq in. as stated in the manual.