That's a great question, and one that city planners and public works departments think about a lot! From my experience walking around cities and looking at municipal guidelines, there isn't one single "most popular" size that fits every street corner. However, a very common and practical size you'll see in many downtown areas is a can with a capacity of about 30 to 55 gallons (approximately 110 to 210 liters).
These medium-sized bins strike a good balance. They're large enough to hold a decent amount of waste from pedestrians without becoming overwhelmingly heavy for sanitation crews to empty. You'll often find them in high-foot-traffic zones like main shopping streets, near public transit stops, and in parks. Their dimensions are usually around 30-36 inches tall (76-91 cm) with an opening width of about 12-15 inches (30-38 cm).
For less busy residential sidewalks or in areas where frequent collection is guaranteed, you might see smaller 20-gallon cans. Conversely, in major tourist hubs or event spaces, larger 65-gallon or even 95-gallon wheeled carts are becoming more popular to handle huge volumes and reduce overflow.
The choice ultimately depends on the specific location's needs: expected foot traffic, collection frequency, and the goal of keeping sidewalks clean and accessible. So, while the 30-55 gallon range is a workhorse standard, the "most popular" size is really the one best suited to its spot on the sidewalk.