I believe you mean the electrode active area. This is not straight forward as CV will tell you about the amount of charge stored during a charge-discharge cycle. The issue is that not all (in your case) the deposited carbon may participate in charge storage. It is common to use planar area which is in your case the area of the GCE* covered with carbon (and wetted by the electrolyte) as your active area. Another method would be to use BET surface area which does not apply to your case. For both methods you have to make the assumption that the active material is fully participating in the charge storage process. Hope this helps!

*Glassy Carbon Electrode

First, a nonfaradaic potential range is identified from CV in quiescent solution. This non-Faradaic region is

typically a 0.1 V window around the open-circuit potential, and all measured current in this region is assumed to be due to double-layer charging.

Based on this assumption

Cdl=dQ/dE=idl/ υ

The Cdl is obtained from a plot of double-layer charging current (idl) vs the scan rate υ. Plotting idl as a function of υ yields a straight line with a slope equal to Cdl. The ECSA of the catalyst can be calculated by dividing Cdl by the specific capacitance of the sample.

Reference: Chem. Rev. 2017, 117, 10121−10211 DOI: 10.1021/acs.chemrev.7b00051

I am highly thankful for such informative answers. My CV curve looks like these. Then in such cases what value for idl i should use? Haixing Gao Either i go for directly line fit and slope or select two points on each axis for Cdl?

You selected potential range is too wide. Typically a 0.1 V around the open-circuit potential.