Are you flushing a pipeline or a piping system to a catch reservoir? Assuming that is the case, you'll need to estimate how long you'll flush the system (both to establish flow and to flush for the required amount of time), what flow rate of water you'll use and the drain down volumes. If you are planning to flush several systems in succession you should be able to take credit for the outflow from the reservoir (which may or may not be significant).
Then, apply a good safety margin. You won't want to have to stop a flushing procedure because the reservoir is overflowing. Of course, if you are flushing several systems and can simply extend the schedule if necessary, you can use a lower safety margin.
If you can supply some more details, you'll get some more specific suggestions.