I suggest you consult an expert of course, but I understand that sometimes you are forced to make some special, desperate, non standard considerations, so, consider the following to give you an idea, hope this helps you to improove your criteria.
If you have not a NFPA13 sprinkler or NFPA 14 water demand and you are not in US, and you have don´t have an authority to define it....
1. NFPA1, Appendix H. Includes a table that defines fire flow(gpm) and durations (hrs) based on type of construction (non combustible, combustible, etc, based on NFPA 220). The minimum is 1500gpm for 2 hrs. This table will lead you to a huge tank, so this table is criticized and is not part of the standard. I´ d use this table if the risk is worth it.
2. Ask fire brigade. The would tell you that the minimum is to operate a mobile equipment for some time. In my country (Costa Rica) it would be 57m3, and they ask for it for installations of 2500m2 and above, when it is not feasible to use NFPA.
3. NFPA 14 has it´s rules that should be applied by experts, but approximately consider that if you need Class II connections (1-1/2 NH), the minimum is two 100gpm hoses working simultaneously, if you need Class I connections (2-1/2 NH), the minimum is two 250gpm hoses working simultaneously, you may have an idea of the water you need if you define a working time, from 0.5hr to 2hrs.
4. There is an old aprox. method, considering area of exposed building with 0.1 to 0.16 gpm/ft2 of water, you define working time for 0.5hr, 1hr, 2hr. So you asume some exposed area. 0.1gpm/ft2 is a flow enough to cool surfaces exposed to fire.
Of course If an expert designs a NFPA 13 building, you will need less water.