- the size of the room,
- ceiling height,
- heat loss through walls, floor, ceiling, doors and windows.
To do this calculation you need to know the "U values" of the various construction materials.
There are online calculaters which will give a rough guide to rad sizes but to be sure it is best to use professional tools. You can do the heat loss calculations room by room yourself manually or down load the free Stelrad Stars programme which is used by professionals.
Want to know more? I found this video on youtube but let me warm you it is a bit long - takes 7 minutes to deliver 3 minutes of info: