Paul,
Wing Length (span) measured in inches multiplied by wing width (cord) measured in inches will usually yield an accurate measurment of square inches.
If wing is taperd, as mentioned above, you need to find your Mean Average Cord (MAC) by adding root and tip cord together and then dividing by 2.
For wing loading: Divide wing area in square inches by 144 to obtain square feet. Then divide weight in ounces by squre feet to obtain wing loading in ounces per square foot.
Example: Typical model of 550 wing area and 44 ounces of weight. To find sq. ft. 550/144 = 3.8194. To find wing loading 44/3.8194 = 11.5201
There is much debate over the best target wing loading; however, Jim Lynch has told me many times "Do not build under 10 and do not go over 14 ounces of wing loading.