That's a great and very practical question! For most standard urban trash cans—the ones commonly provided by municipalities for curbside pickup—standard size liner bags are perfectly fine and often recommended. You typically do not need special ones.
The key is to check the capacity of your bin, usually measured in gallons or liters. Common sizes for city-issued roll carts are 35-gallon, 65-gallon, or 95-gallon. You can purchase corresponding standard trash bags (like 33-gallon, 55-gallon, or 90-gallon bags) that are designed to fit these cans. It's often a good idea to buy bags labeled slightly smaller than your bin's capacity to ensure they don't overhang and get caught during automated pickup.
However, there are a few exceptions. Some cities have specific rules or provide branded bags to encourage recycling/composting. Also, if your bin is an odd shape or a very specific model, a standard bag might not fit snugly. The best first step is to check your city's sanitation department website for any guidelines. If none exist, simply measure your bin's height and circumference, and compare it to the dimensions listed on standard boxed trash bag packages. In most cases, you'll find a perfect match without needing to search for "special" liners.