Possibly the best thing to start with is to check how much heat is required to heat the room. It may give a starting point.
Select this link (available in the projects section) and at the bottom of the page is a BTU calculator. Enter each room details, and it will tell you how much output is required to keep the room warm. Add a little more if you prefer a warmer house.
Google "heatandplumb.com" and search for Radiators & Towel Rails
When you have got the values, visit a website such as B&Q (they have a good selection clearly marked) and compare the output BTU for an equal sized radiator to yours. Remember rads come in three types. Single panel convector, double panel single convector and double panel double convector. Compare like for like.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!