Honestly, the “longest urban bench available” depends on whether you mean a single one-piece bench or a modular system. For a bus stop bench for 6 people, the practical target is 10–12 ft of seat length. The rule of thumb for urban bench dimensions is about 18–24 inches per adult, so six people need 9–12 ft—and since bus stop users like a little personal space, 12 ft is much more realistic.
Most standard commercial outdoor bench models come in 4 ft, 6 ft, and 8 ft lengths. The longest commonly stocked single-frame commercial outdoor bench is usually around 10–12 ft, with some heavy-duty steel or aluminum options reaching 14 ft. These are often sold as park benches or transit benches, and they can seat five to six adults if you use the full length.
If you want a true long run, a modular bench system is your best bet. Modular frames can be connected into continuous runs of 20 ft, 30 ft, or longer. That is often the smartest way to seat six people at a bus stop because you can add armrests, dividers, or center legs and still match the length to your site.
For a bus stop, I would not go much beyond 12–14 ft. A huge uninterrupted bench tends to look empty at the edges, collects trash, and feels less safe. A 12 ft single-frame bench or a 12–14 ft modular run will give you a proper six-person bus stop bench without becoming an oddity.