Before choosing your radiators you must determine how much heat each room requires. Your plumber will be able to give you advice on how to do this.
Most manufacturers supply several types of radiator, like a single panel radiator, or a single panel radiator with convector (an extra fin on the back), or a double panel radiator, or a double panel radiator with convector. When looking on the size charts there will be several radiators giving a similar heat output, but the ideal one is a single panel with a convector as long as possible, but as low as possible. If the length is too long for the space, then one smaller in length but higher should be chosen. Only choose a double panel radiator if the wall space available is really small, if there still isn't enough room then it is possible to get a fanned convector radiator. These have a fan unit that blows warm air out, a small version of this is designed to fit in the kick space of kitchen units.