Shamu.

Please don't write in CAPITALS and not red either. It's interpreted as shouting. Next time I won't respond.

Size is dependant on why it's there. Is it they are just to be used a buffer between in and outflow? Is it there to provide storage for the irrigation of the deep well pump stops?

Could be any size from 1 m3 to 10000. Need to provide some more information about flow rates and how it is planned to operate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

thanks for reply, this raw water tank is to provide storage for the deep well pump stops
and the sequence is the deep well pump will fill the raw water tank and stopped then the sand filter will draw water from the raw water tank and supply water to two separate tanks ( irrigation + flush water ) .

Retention time versus capital cost.

Then it becomes an economic and practical issue depending on your inflow rate and out flow rate.

Also you probably don't want to be starting and stopping your deep well more than say 5 times an hour.

Can your deep well pump run for long periods or is it limited to a certain time / volume before it runs dry?

Is the outflow a constant rate or variable?

Lots of things go into making your decision.
The key for your irrigation tank is simply how long a period do you want / need to store water before you can get more water whilst still flowing out. 5 minutes? 5 hours, 5 days??

First work out some of this items in a design basis then make sure everyone is happy with that then do your sizing work.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Eventho. I don't know the flow rates in and out of the tank between the stoppage period of the deep well pump, your initial estimate would be a tank size at half the amount of water being pumped between stoppage, then you can refine, based on the information presented by the above responders, your initial estimate, once numbers are crushed in. Don't forget to allow additional volume for sediment deposit, minimum water level and freeboard.