To expand on what Bon has already said, in order to find out what size radiator you need in any given room, such things as the floor area, ceiling height, wall construction, window area, insulation in the ceiling, required air changes per hour, target room temperature etc etc etc must be taken into account. These are fed into a calculator which will give you the heat output required for that particular room, and the radiators are selected based on this required output.
So, in short, it is usual to put the most suitably sized radiator in any room, IE the radiator that will put out enough heat to heat the room. Whether it is single or double panel with one or two rows of fins will depend on the available wall area and customers' individual requirements.