That's an excellent question that city planners and business districts grapple with regularly. For a typical busy urban sidewalk—think a main commercial street, transit area, or popular dining corridor—the standard and most effective capacity is between 20 to 32 gallons (approximately 75 to 120 liters).
The 32-gallon bin is often considered the sweet spot. It's large enough to handle the high foot traffic waste—coffee cups, takeout containers, paper—without becoming overwhelmingly heavy for sanitation workers to empty multiple times a day. In exceptionally high-density areas, you might see pairs of these bins to separate recycling from landfill waste.
Why this range? A bin smaller than 20 gallons would overflow too quickly, creating litter problems. Conversely, a bin much larger than 32 gallons can become too heavy to safely lift, may encourage improper dumping of household or commercial trash, and can be visually obtrusive on a crowded walkway. The key is frequent collection; a properly sized bin that is emptied 2-3 times daily is far more effective than a giant, seldom-emptied container.
Ultimately, the "right" size also depends on local factors: collection frequency, pedestrian volume, and the presence of nearby food establishments. The goal is to balance sufficient capacity with practical maintenance for a cleaner streetscape.