I have used optical microscope to determine the area of top platinum electrodes. I was making six spot by sputtering using a mask having hole of sub-mm diameter. Please consult the following paper for the detail
The easiest way to calculate the capacitance density is to divide capacitance by the effective surface area of the electrode. The effective surface area can be calculated using Randles Sevcik equation implementing a simple cyclic voltammetric experiment, using potassium ferricyanide. The slope of the graph, peak current vs square root of scan rate will give you the effective surface area when the diffusion coefficient of potassium ferricyanide in water is known.