Theoretically, a human being can haul a load of almost unlimited size given unlimited time and unlimited equipment. Practically, however, equipment, geography, and time puts limits upon what is physically possible.
Our experience has been that most people can comfortably pull 300 lbs (137 kg) with a typical mountain bike and cargo trailer or cargo trike. How quickly a person can move load of that weight will depend on his or her physical condition. Someone in reasonable physical condition can generally pull a 300 lb load at 10 mph (16 km/hr) on level ground if there's no wind. A person exerting the same effort could pull a load of 600 lb. (275 kg) at a speed of about 8 mph (11-13 km/hr), and a 1000 lb load at about 6 mph.