That's an excellent and practical question! The short answer is: there is no single, universally mandated standard size for the opening on a public waste bin. However, there are common practices, guidelines, and functional considerations that lead to a remarkable degree of consistency you see in parks, streets, and public spaces.
Unlike manufacturing specs for consumer products, public bin design is influenced by a mix of factors. Key among them is the goal of encouraging proper use while discouraging littering and contamination. Openings are typically designed to be large enough for common trash items like food wrappers, cups, and small bags, but often intentionally too small for large bags of household garbage, which helps prevent illegal dumping. You'll frequently see circular or slot-like openings ranging from about 6 to 10 inches (15 to 25 cm) in their widest dimension.
Many municipalities and purchasing agencies follow de facto standards or specifications set by organizations like the American Public Works Association (APWA) or manufacturers' standard product lines. These often prioritize durability, capacity, and ease of collection over a rigid opening size. The design also depends on the bin's purpose: a recycling bin for bottles might have a round opening tailored for containers, while a general litter bin has a wider slot.
So, while you won't find an international ISO standard for trash hole sizes, practical needs for maintenance, public behavior, and bulk purchasing create common, familiar dimensions that make public bins functional and recognizable almost anywhere.