That's a great question, and one that city planners and business owners consider carefully. In most downtown areas across the US and Canada, the most popular and practical size for an outdoor trash can is the 32-gallon (120-liter) to 50-gallon (190-liter) commercial bin.
This size range strikes the perfect balance. A 32-gallon can is large enough to handle the foot traffic of a busy sidewalk or a small plaza without overflowing too quickly. The 50-gallon size is often seen in parks, near transit stops, or in high-density dining districts. They have sufficient capacity to reduce the frequency of collection, which saves on labor costs, but aren't so massive that they become an eyesore or obstruct pedestrian flow.
You'll typically see these made from durable, weather-resistant materials like powder-coated steel, stainless steel, or heavy-duty plastic. They often feature a sturdy, latching lid to contain odors and discourage pests. While you might occasionally spot smaller 20-gallon cans or larger 65-gallon wheeled carts, the mid-range 32-50 gallon is the true workhorse of the urban core.
The choice isn't one-size-fits-all, though. It depends heavily on specific location factors: the volume of pedestrian traffic, proximity to restaurants, collection schedules, and even local wildlife activity. The goal is always to maintain cleanliness while optimizing public works efficiency.