Most model's area is advertised in square inches. At least here in the US by US retailers. And if not, it's easy to measure with a yardstick in inches.

So if you have the wing area in square inches, divide that number by 144 and you've got the area in square feet.

If you know the weight in ounces, the divide that weight by the area in square feet and you got it.

Hope this is simpler to follow.