The maximum permissible trailer mass is usually stated in the vehicle’s technical data; for example, Passat/Insignia models allow around 1,600 kg on average. For vans with 3,500 kg gross mass and a Category B licence, the maximum trailer gross mass is 750 kg; heavier trailers require a BE licence, which allows the combined gross mass to increase up to 7,000 kg.
For safe operation, follow the common recommendation that trailer mass should not exceed 85% of the tow vehicle’s mass (e.g., for a 1,600 kg car, a trailer up to ~1,350 kg is advisable).