"A" that you are asking for calculation, is the area of electrode, assuming that whole of the area of electrode is active and uniformly available for the electroanalytical measurement. If you consider, or are able to calculate somehow that the electro-active area of your electrode is A1 (let), then it would be better and more accurate to use area value as A1 instead of A for diffusion coefficient calculation as it is very sensitive to the area factor and will directly start giving you errors in the orders of tens.