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.

The reservoir is required for a closed loop flusing system.
This system will be used for flushing control panels of piping volume =30 lit. and the flushing pump flow rate= 20 lit/min .The flusing will be in a closed loop .   

As I understand, your system will maintain circulation of a fluid through a system for a period of time using the same fluid.
You need a fluid system similar to a cooling system in a sense that the volume of liquid in the system needs to be large enough as to maintain the pump flow rate with adquate safety margin. Also a volume such that the contaminants' concentrations resulting from the flushing will not diminuish the processes efficiency.
I hope this helps.

Antonio Reis
01 209-834-1900
www.vitrom.com