When I open the plastic casing of electric switch and socket normally used in household like (250 V AC 16 A Switch , 250 V AC 13 A Socket) then I found different shaped copper bar. Copper rod, copper sheet. Shape of these items are not uniform so area is different. So for a certain voltage and current how can I calculate the proper thickness as well as the copper grade. For a cable diameter is uniform and in web there are lots of information how to calculate the diameter of wire but for these copper items I did not found any good reference where I can study.
Thanks in advance and sorry for this stupid question

From the references you've found that tell you how to calculate the diameter of copper wire for a given current, you can set the cross-sectional area of that diameter wire to the required cross-section of the rectangular switch element, assuming the element is made from copper. But in general the metal pieces inside a switch or electric socket are not pure copper because it's too soft. Instead they are typically made of copper alloys such as brass or phosphur bronze. So, you need to increase the cross-section from the above calculation by a factor equal to ratio of the conductivity of copper to the conductivity of the metal you are using in your design.