The size of the radiator is determined by the amount of BTU's (British Thermal Units) required to heat the room. This is done by a simple calculation:

Length * Width * Height of the room and multiply the result by 148 for upstairs rooms or 185 for downstairs rooms.

It is common practice to downsize radiators for bedrooms, and also to increase the size for bathrooms, hallways or lobbies.

If you are using a JavaScript-enabled browser, you can calculate your requirements here - simply enter the room dimensions and click on the appropriate button for the type of room: