The 60 gallon tank is typically used for one running appliance (stove, h20, ect.) not including a furnace.
The 120 gallon tank used for two running appliances (stove, h20, ect.) not including furnace
The 330 gallon tank is used for alternative heating customers ( wood, corn, pellets, ect.) It is also a good size for barn, whole house generator, and pool heater use.
The 500 gallon tank is generally used for residential home heating. This is a standard size tank for multiple appliances and furnace.