Proper thickness of different copper items normally used in Electric switch and socket?
I have never designed a switch or socket, but given the task my starting point would be to look up the appropriate national or international standard for switches and sockets. In Europe this is EN 60669-2-6.

An important factor will be the permissible temperature rise of the internal parts when the switch or socket is used at its maximum current. This will be given in an national standard. From the temperature rise you can work out the maximum resistance of each part. This is quite difficult because you don't know the thermal conductivity of the part in degree C per Watt. There are CAD programs that can simulate this, or you could work it out by experiment. 

From the maximum allowed resistance, and the length of the part, you can then work out the cross sectional area of the copper or brass used to make the part. You would also need to take into account the resistance of each weld or rivet used to hold the parts together. Finally you would test the plug or socket in the way described in the standard and adjust your cross sectional areas accordingly.