CASEY, Ill. (AP) _ Thomas Hannon believes the tiny chocolate chip cookie he perfected in his kitchen is ready to take a big bite out of the nation's $13 billion sweet snack industry.

''All we've done is miniaturize the most popular cookie in this country and make it snackable,'' said Hannon, who will begin nationwide distribution of his new ''Tribbles'' this summer with production estimated at 1.8 million pounds monthly.

The crunchy cookies - some with mint, peanut or malt added - aren't much bigger than an ordinary chocolate chip. They sold well in grocery candy aisles during 1985 test marketing in Wisconsin.

''We've stepped out from the crowd,'' said Hannon, founder of Bitesize International. ''You can name a lot of candy bars, but Tribbles are unique.''

Hannon spent 15 years as a salesman and executive with candy companies, marketing such snacks as M&Ms.

But he decided consumers wanted to pop something into their mouths with the lightness, crunch and nutrition of a cookie rather than candy. With fewer calories and more pieces per bag, Hannon, 42, figured such a product would offer ''a longer eating experience for the money.''

It was his employer - M&M-Mars candy company - that persuaded Hannon to start his own business. His reward for leading the Chicago division to a great sales year was a necktie ''that didn't go with anything.''

''It put the lights on for me. If I can do this for somebody else's company, why can't I do it for myself?''

The tie is framed on Hannon's wall with the inscription, ''It takes more than a four-in-hand to motivate people to excellence.''

After a stint with a Southern Illinois candy company, Hannon quit to research the market for a tiny snack cookie and develop the formula.

''I went through 75 or 80 recipes, trying each one on my family, friends, neighbors to see what they liked and what they thought should be added,'' said Hannon, of Mount Vernon.

After perfecting the cookie, Hannon found a name - Tribbles: TRI ''from what we hope people will do'' and BBLES ''from nibbles''.

He invested $525,000 and began looking for the $3 million he would need to get Bitesize International into full production.

Clark County offered financial help to bring Hannon to Casey. Bitesize International put its headquarters in a downtown office in May 1985 and shipped out its first Tribbles two months later.

Baking of the bite-size cookies is done in Atlanta by the Keebler Co. - ''the elves and Tribbles work well together.'' Batterlite Whitlock Co. does the packaging in Springfield.

Eventually, Hannon hopes to build his own packaging and shipping complex in this southeastern Illinois community and perhaps bake the cookies locally. He expects employment to rise from the current 27 to about 300.

''We're going to see copycats. That's why we're getting out there in the market fast,'' Hannon said. ''I'm no gambler. I know the snack market, and I believe in this.''