Gary,
Cooler size should take several issues into consideration. As a rule of thumb larger is better if you have an inline temp controled valve or method to control diversion to cooler and can tweek by blocking some off to airflow in cooler times. Without control any size is likely to large for many engines. You want the
oil to reach about 215 degrees and then cooler to be able to keep it there and not above about 240 degrees.
Oil is intended to be most efficient at temp and needs to reach at least 210 so that all outgassing will take place to keep
oil clean as possible and most viscous to lube properly. Water condensate and fuel will collect in oil below vapor point of water.
Too large without control will not allow needed temp. Too small or not enough cooler if stuck in traffic, live in HOT climate etc and type of driving all decide how big is needed.