That's a great question! It's something I've actually wondered about myself when taking out the trash at the park. From what I've seen and from talking to folks in parks departments, there isn't one single "standard" size that fits all parks everywhere. It really depends on the location and expected use.
However, the most common size you'll find for a standalone outdoor trash can in a typical city park is usually in the 20 to 50-gallon range. The 32-gallon can is a very popular middle-ground choice. You'll often see these as classic round or square metal cans with a lid, or the newer plastic models.
For high-traffic areas like near picnic pavilions, sports fields, or playgrounds, parks will often use larger containers. These can be 65-gallon wheeled carts or even small dumpster-style units that hold several cubic yards of waste. In more secluded spots on a trail, you might find a smaller 12- or 20-gallon can to handle lighter use.
The main goal for park managers is to balance capacity with maintenance. A can that's too small will overflow quickly and create litter. One that's too large becomes incredibly heavy to lift and empty. The 20-50 gallon standard is a good sweet spot—it can hold a fair amount of trash from a day's visitors but is still manageable for a crew to service daily or every other day.
So, while it varies, if you picture a typical park trash can, you're probably thinking of one that holds roughly 30-40 gallons, or about 4-5 standard kitchen trash bags worth of waste.