Don't snicker, but Mars Inc. just increased the size of its candy bars without raising the price.

Mars, the McLean-based company that manufactures such confectionary best sellers as M&M's and Milky Way, Snickers and 3 Musketeers bars, announced yesterday it was increasing the size of its candies from 7 percent to 13 percent without increasing the price.

"We're taking a calculated risk that gains in sales volume in these large items will give us improved productivity and thus enable us to cover our cost increases for the forseeable future," said Ken Clark, the man from Mars.

In case things don't quite work out that way, however, Mars announced a "modest" price increase for all its other candy products such as the large-sized package goods containing the same candies. Just 5 percent, a spokesman said.

Clark said the candy bars involved in the size increase make up two-thirds of the company's candy sales.

That's the good news. The bad news is that the consumer may never realize the price benefit. Because of legal restrictions, Mars does not suggest a retail price for its products. That decision is left up to the individual retailer, who is under no obligation to charge the same price for the new larger bars.

Clark said Mars charges its distributors 16 1/4 cents for each candy bar and the average retail price for the candy bars was 25 cents.

He said the price increases for the other Mars candy products were necessitated by an average 10 percent increase during the past year for raw materials such as peanuts, sugar and milk. "These factors have made it necessary for us to announce a modest price increase in our large-size package goods, but we have made a firm decision to hold the line on prices of our major singles bars, whose weights have just increased," Clark said.

In other developments on the candy front yesterday, striking chocolate workers at Hershey Foods, along with Mars one of the nation's largest candy makers, reached a tentative agreement on a new contract. The 3,000 workers have been on strike for 17 days. A vote on the new contract proposal is scheduled for Friday.