you can use any shape whose area and thickness is known. Preferably thin but large area so that even if conductivity of your system is not good, the impedance remains with in measuring limit of instrument.

You can use cylindrical pallets of very small thickness.

Depending on your instrumental specification you can use any shape, preferably most of the instruments use cylindrical pellats of very small thickness

what is the formula of and parameters of ac conductivity measurement using LCR meter...and what is the difference between the measured capacitance from LCR meter and CVs.??