What is the maximum permissible length of dimension X?

Your answer is correct. According to German traffic regulations, the maximum permissible width dimension marked as "X" for vehicles or their loads is typically 3 meters. This means the widest point of a vehicle or its load, as shown in the picture, must not exceed 3 meters to ensure safe passage on public roads, avoid obstructing traffic, and reduce the risk of accidents. Staying within this limit is important for compliance with road safety standards and legal requirements.