THE REAGAN ADMINISTRATION has now defined, in extraordinarily dramatic terms, its anti-trust policy. By settling the AT&T case, and dropping the proceedings against IBM, it has provided its own judgment on the two great anti-trust trials of the past decade. No doubt this policy is derived in some part from the administration's ideological attitudes toward big--and very big--business. But there's more to it than that. American views toward anti-trust regulation have been moving in this direction for some time, and not only among the Republican right. Even people who remain uneasy about great concentrations of corporate wealth have had to acknowledge that the nature of competition has changed.

Until the late 1970s, Americans thought of foreign markets and foreign competition as peripheral and incidental to this country's enormous continental economy. That was wrong; foreign trade was already playing an extremely important role here. But it was a well-entrenched habit of mind in American law and politics. How big is too big? The relevant yardstick seemed to be a company's share of the national market. The traditional view still held in the late 1960s, when the IBM case was launched, that a company with 70 percent of the U.S. market for computers enjoyed a scale and dominance that made it a menace to its smaller competitors.

But with the enormous expansion of international trade in the 1970s, Americans learned that the domestic market share isn't always the only thing that counts. Automobiles and steel are, by the conventional measure, both highly concentrated industries, but no one would argue that either suffers from a lack of competition. It comes from abroad. The same thing is increasingly true in the field of computers, communications equipment and data processing.

Mr. Reagan's Justice Department says that it will prosecute vigorously any cases of specifically anticompetitive actions by businesses. But being big is no longer, by itself, a criterion. That's a defensible position, and even a necessary one. But as a preview of the future economy, it's not wholly reassuring.

For governments--as the coming debates in Congress will demonstrate--there's the distressing issue of trying to regulate international business with national standards. Every country says that it wants open competition, but no government wants to see its companies get left behind. None of the major industrial countries is likely to allow itself to be left without at least one company in computers and telecommunications. Worldwide competition may have made the traditional American measure of market shares obsolete. But no two governments hold quite the same ideas about the regulation of that competition--especially among the high-technology industries on which they expect their future prosperity to depend.