It doesn't matter how big or small your business is, as certain agreements are illegal regardless of your justification. More importantly, they're criminal. Here, size and position in the market matter. Interestingly, even small businesses can dominate a market if the product is unique and the market is small. In addition, mergers and acquiring ongoing business assets may require government authorization before the deal can close.
Some industries are more prone to antitrust challenges than others.
Antitrust troubles come more often to businesses emerging from regulation and duopolies and oligopolies where a market is controlled by a small number of companies. If any of these conditions apply, you'll want to be especially alert to antitrust concerns. The first line of defense is knowledgeable employees. It's wise to provide frequent training for all managers, especially those in sales, marketing, strategy, deal making and procurement. Short vignettes adapted to your industry and which resemble recent cases, along the price tags fines, attorneys' fees, jail terms of those cases make a lasting impression.