hi
ducati is quite right
but if this becomes an obsession you can do some research on the net regards radiater sizing or go to a supplier of central heating supplies and you can purchase some form of "matrix"device to work out the correct size
bob
In fact this site in the projects section has a link to a radiator calculator.
What you need to be careful of is not to unbalance a system by fitting larger rads in some rooms and not others. If you do, some rooms may become very warm whilst others are cold.
If you type the key words of your question into our search box to the left of the site you may find the answer is already posted or is in the DIY projects section of the website. Every post goes through a monitoring process and using the search box may speed up your answer.