Proper sizing of a septic or a cistern tank depends upon a number of factors. For septic tanks, average sewage flow is the most important consideration. Most states require a minimum septic capacity of 1000 gallons. For cistern tank sizing, you need to determine your home water use. A good website to research what size is best for you can be found at http://inspectapedia.com/septic/Septic_Systems.php. For cistern tank sizing, consider the home water use calculator at http://apps.auroragov.org/WaterConservationCalculator/Default.aspx.