What is the smallest/largest electrode spacing I can use?

Short Answer: Your electrode spacing can be as small as a centimeter apart or as large as several miles/km long.

Long Answer: Surveys can be a few inches/centimeters to several miles/km long. Often lab studies are done on scale models such as water tanks with electrode spacings of a centimeter. Deep mining and Geothermal studies have electrode spacings of 100m or more.