BS5588 pt 11 and 12 gives some guidance.
I have also used common sense in the past- worked out how many jets may be used to fight a fire, allowed 1000 litres per minute per main jet, sustained for 1.5 hours. This is well over and above the 5588 formula but does include a safety margin. My last project, a 50000 sq m warehouse provided a 378 cubic metre tank for firefighting based on the above- to augment a poor fire hydrant supply. In the event it involved a tank the same size as one of the pair of sprinkler tanks and made it possible to use an area of otherwise derelict land and create 1500 jobs.
Take care on the design of the outlet from the tank to ensure it will deliver the required flow, and access for firefighters / prevailing wind.
There is another document that gives definitive advice I think produced by cacfoa. Will look in my archives over the next couple of days to see if I can find it.
