Look in NFPA 13, 13D or 13R depending on the structure. All sprinkler systems must be designed by a design professional/ sprinkler engineer who calculate spacing, flow requirements and storage capacities for those systems "off the grid".
If you look at ISO requirements for water supply points, they require that a water supply point that is not connected to a main should be capable of providing a fire flow of 250 gpm for a minimum of 2 hours (120 minutes). In other words, a rated water supply point should have a year-round capacity of 30,000 gal (250 gpm x 120 minutes = 30,000 gal). This would apply to a pond, cistern, or tank. 250 gpm is a relatively low fire flow for a structure fire, but most buildings don't require a full two hours of non-stop water flow, either!
My fire district conducts operations using tanker shuttles, with key water source locations throughout the district. My observations, based on our last several years of operations, is that a typical residence that is fully involved upon arrival seldom takes more than 6,000-10,000 gal of water to extinguish. [Your mileage may vary! ;-) ] I'm sure that any fire department would appreciate a supply of water on scene, providing they can access that water. You might consider having a large, valved opening on your tank. Check with your local fire department to see what type of threads they use, hopefully National Standard Thread (NST), and what size of valve they would prefer to use to access the water. You might find that the local fire department would contribute to your project if you made your tank of water available for other emergencies in your area.
You may not be interested in supplying 30,000 gal of water by yourself, but it is a recognized amount based upon a reasonable set of assumptions. You can modify this design based upon your own set of assumptions and the input of your local fire department.