If you fill your system, you can make any size you want since the water is circulating.

In hydraulics volume of the tank should be be more than 3 times the flow rate in LPM. That is if flow rate is 20 litre per minute, capacity of tank should be more than 60(20*3).

for your system it may be 8.5*60*3 lit

The correct answer depends on the function of the tank. For example, If the tank is for thermal reasons, you need to look at the thermal effects and consider cost and size. If there is hydraulic capacitance involved, tank size and construction is very important. If the tank is for mixing, that is a different question and answer.

Dear Pradeep,

Thanks for the answer, But i am looking for a minimum capacity of a tank that i can have. The tank is just for holding the water. If the pump pumps back 8.5 lt of water in 1 sec back into the tank. Can you justify why we need 3 times the flow rate of pump in LPM?? ( Even if we have a 100lt tank capacity, then in each second 8.5lt would be filled in the tank, same time 8.5lt would be sucked by the pump right?

Dear Scot ,

As answered by xianglei, I am just using the tank for filling and recirculation, and not for anything else. So can you justify your answers on which will be the appropriate tank size for me..Thanks...

what is the hold up volume of the pump and piping? To avoid running out of fluid, you need to know the volume in the pump and all the lines and the levels if the tank is to be open.. If it were just 'recirculation', and there is no other function, why have a tank at all? Connect the pump to itself. Volume is volume of pump and tube.  If there is a mixing function, then you need to look at the mixing speed required.  If there is a heat exchange function, etc....

you must be an undergraduate student.

I given you capacity of tank according to requirement of hydraulic system. We need more fluid for the heat dissipation and settling of dirt. In your project tank capacity is depend on purpose of circulation  water.