Small-scale thinking never has been a trademark of Dr. Armand Hammer, the multimillionaire philanthropist-oilman, but yesterday a couple of U.S. senators suggested he may be thinking too big.

Hammer went before a Senate subcommittee to aver that with the country in "the greatest crisis since the Republic was founded,"it is time for a crash program to convert oil shale to petroleum.

The board chairman and chief executive of Occidental Petroleum Co. said the shale of Colorado, Utah and Wyoming holds the answer to solving the nation's oil shortages.

With proper federal financial support and tax incentives, he said, his company and others could be producing 2 million barrels of synthetic petroleum a day from oil shale by the 1990s.

Moreover, Hammer said, the private firms could turn out synthetic fuel at a cost ranging between $15 and $25 a barrel - making it competitive with imported petroleum.

"This is no time for timidity. We've got to be told. It is no time to quibble over $3 billion of federal money when we spend $65 billion on imported oil. The government can risk $3 billion. The money will not be wasted," Hammer told the subcommittee.

But before the day was over, the only subcommittee members present, Sen. Wendell H. Ford (D-Ky.) and J. Bennett Johnston (D-La.) gave Hammer and his optimism a hard going-over.

Through questioning of Hammer and other witnesses, including oil shale promoters and environmentalists, Ford and Johnston raised doubts about the Occidental chairman's statistics and approach.

Ford, whose preferences for synthetic fuel development lie in the coal fields of his home state, remembered that Hammer only two years ago was saying oil shale could produce synthetic petroleum for $8 to $11 a barrel.

Ford wondered, why the $25 a barrel, figure today?

"Inflation," Hammer said. "the longer we dilly-dally, the worse the inflation will get."

Hammer left the impression that his company and others, given the proper financial backing by Uncle Sam, could almost immediately leap into production.

But another oil shale developer, Richard M. Lieber, executive vice president of Rio Blanco Oil Shale Co. of Denver, took exception under questioning to some of Hammer's assertions.

He indicated that the 2 million barrel goal will be hard to reach by the 1990s and suggested that far more of the West's precious water will be needed for oil shale development that Hammer indicated.

Ford told Lieber, "Dr. Hammer's gun-ho, but you've described some of the problems that most of us are going to look very hard at before we spend that kind of money - the $3 billion to get this going."

Yesterday's hearing was part of Senate deliberation on legislation designed to put more federal support quickly into synthetic fuel development.

The pending bill, introduced by Sen. Henry M Jackson (D-Wash.) and a bipartisan group of senators, proposes a more modest approach to subsidizing oil shale than Hammer espoused.

John McCormick of the Environmental Policy Center cautioned that if Congress listens too closely to Hammer, it will end up "spending many billions of dollars on synfuels subsidy programs which are unrealistic, poorly conceived and offer no solutions to our immediate problems."

A better approach, he said, lies in Congress changing "from its obsession with increasing supplies to a renewed dedication to decreasing demand."