Tommy Hearns rises to 6 feet, 2 1/2 inches. As a mere welterweight, he has a 78-inch reach that would have given him an advantage over both Jack Dempsey and Gene Tunney. He is unbeaten after 32 professional bouts and his formidable knockout record is unmatched by any fighter in history.

So fierce is his right-hand punch that his 30 knockout victims can be divided into two groups, those whom he flattened quickly and those who finally became the objects of a referee's pity. In the ring, Hearns has been the very embodiment of force. For good reason they call him "The Hit Man."

Hearns will meet Sugar Ray Leonard for the total welterweight championship of the world Wednesday night in Las Vegas. And Leonard probably will lick him. Very possibly knock him out. That view is premised on the belief that Hearns never has fought the likes of Leonard, that he will be disagreeably surprised, will find himself clearly overmatched and have his first experience as a loser.

On that night in Vegas, "The Hit Man" will discover that some fighters are harder to reach with his right hand than others. Also, that Leonard, too, can hit, if not with all of Hearns' raw power.

Hearns will not be in there playing solitaire. There will be another presence in the ring, a man with skills, who has also knocked out 21 guys. Leonard, no fool, may be of no mind to slug it out with Hearns, but at times of his own choosing he will be throwing at "The Hit Man" the right hand that did take Wilfred Benitez and a few others out, with one delivery. And Sugar Ray's left does not hang limp.

It was last June 24 when both Hearns and Leonard fought different opponents on the same card in Houston that my mind was made up about Hearns versus Leonard. When the night ended, one could take a ticket on Leonard instantly. He had licked a junior middleweight champion, the rugged Ugandan, Ayub Kalule, who had won 36 in a row and had never been knocked off his feet as a professional. In the late rounds, Kalule finally asked the referee to stop it, a la Duran.

Meanwhile, Hearns was knocking out Pablo Baez, a zero who had been knocked out eight times before by other guys. It was only by some hanky panky by one of the boxing associations that Baez had gotten onto the card. He was mysteriously promoted to No. 9 , a "rating of convenience" to help the promoter. Previously, Baez (14-8-2 before the fight) had been unranked.

It may look good in the records that Hearns stopped Baez in four rounds, with one of the fierce right hands that is his trademark. But he wasn't beating much and for three of those rounds Hearns looked ineffective, and wooden in his movements. There had to be a rapid understanding that the fast-moving Leonard would eat this kind up. Hearns as well as Baez. Except for the threat of his punch, Hearns would not trouble Leonard much.

Randy Shields, who went 13 rounds with "The Hit Man" last April before being stopped on cuts has something to say about Hearns' right hand. Shields opines that "Hearns telegraphs his punches." If so, Leonard will not be home to receive such messages.

Leonard, who now has half of the welterweight title, wants all of it. He is infatuated with owning the title, with his reputation as the biggest box office draw in all boxing history, with those TV commercials that bring him more money and lets him beam his joy at having his little boy on the air with him. It's the good life, too, when the networks ship him everywhere at good pay to be a commentator for other big fights, and expand the visibility he likes so much. He doesn't want to say goodbye to all those good things.

In the ring, Leonard may appear to be biding his time, but the killer in him is never subordinated. The light punchers last longer with Sugar Ray, but he wants no truck with the dangerous kind like Hearns. He doesn't like punchers to hang around very long.

As for that big advantage in reach Hearns will have over the shorter Leonard, it's meaningless against good fighters who choose to stay away or have the ability to take a punch in order to get inside. Exhibit A was Rocky Marciano. Everybody he fought had a big advantage of reach against him. When Marciano retired he was 49-0. He knew his shorter arms were an advantage inside, their longer arms their handicap. Leonard fights well inside and, like Marciano, has the good chin that can get him there.

Visually, the shorter Leonard will be measuring all of Hearns' 6 feet, 2 1/2 inches when they get in the ring. And he will doubtless have the same thought that occurred to Whitey Ford when that pitcher, on viewing for the first time the 6-7 Frank Howard in the batter's box, exclaimed, "Jeez, what a strike zone."

How, exactly will Leonard deal with "The Hit Man?" The best answer I know is to cite all of Leonard's tools, including his quickness; the savvy he has soaked up in all of his 31 fights against tougher men than Hearns met; his own good punch, and his own ability to take a punch if necessary (23 rounds against Duran). Just put it down that on Wednesday night, Leonard will, in State Department talk, take all necessary measures.