I just did one of my favorite tests with my oscilloscope. Touching my finger to the signal lead only, I see 2V amplitude of 60 cycle hum. (This is in a normal apartment residence with some appliances running.)
Since the scope has 1 megohm input impedance, it implies 2uA of current traveling through my body. Supposedly I pick this up via capacitance from house wiring. There are the basic raw calculations if you are looking for a Farad value. Probably in the pF range as suggested in post #2.