Short Answer: Your system size/electrode requirements will be dependent on your depth of penetration and your resolution requirements.
Long Answer: Before purchasing or renting equipment, be sure to know a few key elements of your survey (listed below). This includes depth of penetration, resolution, and a basic understanding of your survey's geography—such as topography and the size of your working area.
Understand Your Survey Site's Geography
Though the desired depth of penetration and resolution will help you determine your electrode/system requirements—knowing the basic layout of your survey area first will help you understand what limits you may have to get your desired depth. This can also include topography, which you can read more about it in this article: Do I need to account for topography?
Know Your Desired Depth Of Penetration
The depth of penetration is dependent on how far you are able to spread out the available electrodes in a straight line. You can usually expect a depth that is roughly 18% of the distance separating the two dipoles. Please see this article for more on this topic: How deep beneath the surface can I see using Electrical Resistivity Imaging?
Know Your Desired Resolution
The best resolution your resistivity/IP scans can achieve is dependent on the electrode spacing. Following the Nyquist-Shannon Sampling Theorem, the resistivity method can only detect resistivity changes larger or equal to half the electrode spacing. Please see this article for more information on resolution: What resolution can I expect?
Know Your Desired Survey Timing
With a longer electrode cable, you can always perform quicker and more resolvent surveys. Keep in mind that longer cables will also mean more weight. Our standard systems come with a 6-meter (20 foot) electrode spacing. However, if most of your work is shallow, then you should consider shorter cables.