Feeding the world. As the UN marks World Food Day, mankind faces the perplexing spectacle of both famine and surplus. Future farms: bigger, and possibly better, food suppliers

TODAY'S roast beef sandwich was produced by ... whom? If you guessed the American farmer, you're right, of course. But hold on. What kind of farmer do you envision? Is it one of those Texas-size operators with air-conditioned offices, a farm manager, and dozens of hired laborers? Or is it a small, Midwestern farmer, whose family lives and works on the operation?

Chances are, both types of farms had a hand in today's lunch. The lettuce may have come from a tiny family operation in southern New Jersey, the sandwich-bread wheat from a corporate farm in Kansas. The beef might have been raised by a midsize farmer in Missouri - or a huge California outfit that handles every step from raising the cow to packaging a steak.

For years, this double image of agriculture - the corporate giant and the family farm - has blurred political debate on farm issues. These issues reach across national boundaries and pose a startling question for the world's consumers: What kind of farmer should produce our food? The most efficient one? The farmer who is the best steward of the land?

The debate over these issues has never been resolved. Nevertheless, a consensus has formed on the impact of agriculture's current direction. Among the trends:

As the number of farms declines, the average size is increasing - not only in the United States, but in the rest of the developed world, too. For example, excluding the very smallest operations, the average size of a Belgian farm doubled between 1960 and 1980. The trend is not new - civilization has progressed because fewer and fewer people were needed to produce food.

``That's an inevitable trend,'' says Allen Shiau, an economist at WEFA Group-Wharton Econometric Associates, although it is happening more quickly in the US than in Japan, for example. Exceptions to the trend are the Soviet Union and other communist nations that operate huge collective farms, and the few developing nations, such as Nicaragua, that have broken up huge farms and put land in the hands of peasants.

``There's been continual increase in the numbers,'' says Ken Krause, an agricultural economist with the economic research service of the US Department of Agriculture. In 1969, US corporate farms accounted for 15.3 percent of agricultural sales. By 1982, the total had jumped to 23.3 percent. On the other hand, 9 out of 10 of those corporations are controlled by families, Mr. Krause says, many of whom incorporated their farms for tax purposes.

Outside the US, these trends are not so clear. In Western Europe, corporate farms are virtually nonexistent, says Jacques Vonthron, first secretary of the European Community's delegation in Washington. ``The size of the farm is too small to have shareholders,'' Mr. Vonthron says.

In many developing nations, particularly in Central America, the opposite is true. Large landholders often wield enormous political and economic clout, observers say.

``I don't think it's a serious problem - in fact, I don't think it will ever become a serious problem,'' says Fred Sanderson, senior fellow with the food policy center at Resources for the Future, a research organization in Washington, D.C. Even with the current difficulties, at least 450,000 US commercial farms are expected to survive. ``That's a far cry from the oligopoly we have in most industrial sectors,'' he adds.

More efficiency, hidden costs

But the trend worries Kevin Danaher, a senior analyst with the Institute for Food and Development in San Francisco. ``If someone, of their own volition, chooses to leave the farm and enter the city, that's fine. But this [situation] is people being forced by economic and political power off their land. It is largely economic forces that are tending toward economic concentration.''

What does the concentration mean? Economists point to a more efficient, market-driven industry and continued low food prices for consumers. Food prices are much lower in the US than in Europe, for example, where farms are much, much smaller.

Mr. Danaher, however, sees hidden costs of large-scale, market-driven farming. ``What you find is less employment, degradation of the environment, and less democracy,'' he contends.