Math Challenged
I happen to have a rudder in the garage, but can't remember the formula for the area of a trapazoid. Probably can't spell it either, but the measurements turned out to be 29" across the top and 18" across the bottom. The leading edge is 48" and the trailing edge is 54".
Using a formula found with that trusty tool Google I come up with 1128 square inches, but it also told me that a trapazoid has two parallel sides which the rudder does not. That means the small triangular area at the top of the rudder is not included since I used the non-parallel top and bottom as the base figures and the verticle leading edge as the height. A calculated guess at the areea of the triangle being 87 square inches leaves an estimated rudder area of 1215 square inches.
That may be close enough for your purposes, but perhaps you should just give Yves the measurements. If anyone else out there has had a math class since 1962 you might tell Captainspicy and I how close I came.