If you know how much liquid you want to store, just make a tank that size.  Design for water, equip and operate for crude.  No real biggie but there are a lot of little details and regulatory issues that you might miss.  Best to get the help of an engineering firm that does this all the time.  If you are in the US, there are plenty.  If you are doing the actual separation in this vessel, an oil water separator may be better and more effective.  You'll still need a tank or two before and/or after the separator.  If you are thinking of using this tank for the separation, the flow rate, level of contamination, temperature, etc will be needed.