How long is a semi trailer and truck?

Both power units and trailers come in many lengths. Bear in mind, a semi-trailer is simply a trailer which does not wholly support its own weight - thus, boat trailers, equipment trailers, etc. are semi trailers. But, for the sake of argument, we'll assume you're referring to a semi-trailer of the type intended to be pulled by a Class 8 truck and a Class 8 truck. Again, both come in varying lengths. Most commonly, 48 and 53 ft. trailers are seen, with a power unit which has a wheelbase between 220 and 270 inches (for a sleeper berth tractor). So you'd be looking at between 65 and 77 feet - some combinations are longer, some are shorter - but this is the "norm".