Area = Length * Width

However, the surface of an airplane's wing isn't flat. This will require a little more. The easiest way I can think of to figure it would be to use a piece of string. Hold one end of the string on the leading edge. Pull the other end to the trailing edge. Mark each end. Measure the distance between the marks. Use this as the width. Measure the length. WingArea = 4 * (width * length) - assuming a symetrical airfoil.

If it's a tapered wing with a symetrical airfoil:
Use string method at the tip and the root. This is a trapezoid. Area = [(TipWidth + RootWidth) / 2] * length
WingArea = 2 * (TipWidth + RootWidth) * length

A flat-bottom or semi-symetrical airfoil will require a top wing measurement and a bottom wing measurement.
WingArea = 2 * (TopArea + BottomArea)