Heat output
To choose the correct radiator you have to ask one question:
Does your radiator need to heat the whole room or just dry towels?
As a guide:
The bigger the radiator surface, the higher the heat output and the larger sizes may be neccessary if the room has to be fully heated.
Examples:
For the larger room when heating and towel drying are both required.
For a significant design statement and high heat output.