Far reaching antitrust legislation that would ban mergers between large corporations simply because of their size has been drafted by Sen. Edward M. Kennedy (D-Mass.), who is scheduled to become chairman of the Judiciary Committee.

Under Kennedy's proposal, which will be introduced within the next few weeks, the government for the first time would preclude mergers simply because of size in an effort to deal with the problem of economic concentration.

Kennedy's bill would:

Prohibit all mergers involving the 100 largest American companies, those having $2.5 billion in either assets or revenues. As an escape clause, companies at that level could merge only if they were willing to spin off assets of comparable value.

Ban all mergers between two companies in the top 500 as well -- those with at least either $350 million in revenues or $200 million in assets -- unless they could be justified by proof of substantial economies of scale or other efficiencies.

Bar any firms in that same top 500 category from acquiring any "leading" firms -- companies that have 15 percent of a concentrated market. Large companies wanting to diversify into a new business would have to acquire a smaller firm or start from scratch.

The proposed legislation would keep intact the current antitrust laws for all other firms; they test the legality of mergers and acquisitions in terms of their anti-competitive effect by barring combinations that may lessen competition or tend to create a monopoly.

The Kennedy plan would not require proof of anticompetitive impact to stop a merger between large companies. The implicit assumption is that large size alone is troubling for economic, political and social reasons.

Mergers between large companies that have none of the same lines of business -- called conglomerate mergers -- have been difficult to challenge under current antitrust laws because an effect on a market or markets cannot be shown easily. But concerns have been raised by many that those mergers increase concentration of economic power in fewer and fewer hands.

Data collected by the Federal Trade Commission indicates that the percentage of manufacturing and mining assets accounted for by the top 100 industrial corporations increased from about 44 percent in 1955 to about 53 percent in 1977.

Kennedy's antitrust bill is "one of his top two or three legislative priorities this year," according to David Boise, who will be staff director and chief counsel of the Judiciary Committee. Kennedy expects to schedule hearings before the full judiciary committee in early February.

Although Kennedy plans to move ahead with the measure, his staff has been working with the White House, the Justice Department and the Federal Trade Commission in search of a consensus position. So far no agreement has been reached on a bill.

A draft measure drawn up by the Justice Department's Antitrust Divisions takes a slightly different tack by barring any future mergers that would result in a company with revenues or assets of $2 billion or more, where each firm has at least $100 million in sales or assets. It would also bar acquisitions by a $1 billion company of a firm with at least a 20 percent share of the concentrated market, where that market totals at least $100 million in annual sales. The mergers could be a lowed if they result in the substantial enhancement of competition.