That's an excellent and very practical question! The short answer is: it depends entirely on your specific trash can. There isn't one universal "standard" size that fits all outdoor bins, as they come in countless shapes and capacities. However, many common-sized liners are widely available.
Most rectangular or round outdoor bins from common home improvement stores will often correspond to standard bag sizes you find in rolls, like 30-gallon or 55-gallon bags. The key is to check your bin's capacity, usually listed on a sticker or in the manual, and then buy bags labeled for that gallon size.
You might need "special" or non-standard liners if your bin is an unusual shape (very tall and slim, square, or has a tapered design), is extra-large (over 65 gallons), or is a built-in/public space receptacle. For these, measuring is crucial. Use a tape measure to find the height (top to bottom), the perimeter or width at the top, and the depth. Compare these to the bag dimensions on the product box.
My advice? First, measure your bin. Then, buy a small roll of a standard size you think might fit. Test one. If it's too loose, slides down, or is too tight, you'll know to adjust. For durability, always opt for "heavy-duty" or "contractor" bags for outdoor use—they resist tears and weather better. A well-fitting liner makes cleanup a breeze!