SPICE - How model body for EMI in a EEG system
View the body as a square meter of conductive material, placed 1 meter above a conductive concrete floor.

Use the formula for capacitance, suitable for parallel-plate modeling and to get an approximate answer for many other topologies

C = 8.89e-12 Farad/Meter * air=1 * Area/Distance

For the 1 meter cube we just described, your C is 8.89pF