Wing area is based on the overhead view "plan form" of the wing. If you cover that with 1 inch square graph paper and count up the squares covered (adding partials by % covered) you get the wing area for a monoplane. (biplanes/triplanes you have to do each wing separately and add them)
You'll get the same number by breaking it up into rectangles and triangles, using basic geometry and adding.
Weight/area = wing loading.
Pick the chord entered to get the actual wing area at the actual span.
400 sq in wing at "just" 5 lbs will have a HIGH stall speed... 16 lbs is way out.
You may be trying to put too much power in too small of an aircraft and making a very large prop driven car. It looks to me like you need 1/4 of the power you are sticking in the airplane.