IN RECITING his credentials for high federal office, former California governor Ronald Reagan frequently mentions that, if California were a separate nation, its economy would make it the seventh largest nation in the world.

The present California governor, Edmund G. Brown Jr., who last week returned to his home state after withdrawing from the presidential race, referred over the weekend to going "back to the biggest state, which is the eighth largest nation in the world." We have also seen California listed in other reports as the sixth and the ninth largest economy in the world.

This is the kind of question we like to believe we are here to answer so that nobody will lose either sleep or wagers over the ranking of California if, in fact, California were a separate nation.

The largest nation in the world, in economic terms, is the United States of America. The gross national product for the last full year for which figures are availalble was $2,088.2 trillion. The Soviet Union was a distant second at around $700 billion. West Germany won the international show position in the gross national production competition with an annual figure of $636.17 billion. In fourth place were the Japanese at $564 billion, followed by proud France at $471.13 billion and the populous People's Republic of China at approximately $340 billion. Those are the big six: U.S.A., U.S.S.R., West Germany, Japan, France and China.

Next on the list, in front of the United Kingdom, Italy, Spain, Brazil, South Africa, Netherlands and all the OPEC countries, is -- you guessed it -- california. If California were, in fact a separate nation, its gross (no wisecracks, please) product of $295.5 billion would make it the seventh largest national economy in the entire world. Mr. Reagan and Brown (who uncharacteristically understates the case) cite their figures as evidence of their experience in managing a very large and very complex economy. But these amazing numbers suggest . . . we don't want to be too ungracious about it . . . that California was off and running to its world-class ranking before either of these candidates was runing California -- let alone running for national office.