Size of radiators

[chrisdan]

How can I calculate the optimum size of radiator (central heating) needed to heat a specific size of room?

[woodchopper]

Try this -
http://www.radcalcs.co.uk/

[buildersmate]

Good tool that. Just be aware that it says 'average heating requirements. If you have a very modern house with better insulation in walls and roof, your requirements will be less. And vice versa.
If you oversize a rad, you can just let it self-regulate (using a TRV). If one undersizes it, one is stuffed for ever. That's why plumbers err on the large size.