If you wish to change old radiators and discover you cannot find the same size do not despair. In Britain like everything else radiators came in imperial sizes(inches) but are now sold in metric (centimetres). The changeover was in the 80s so if you wish to change your radiators rather than lift floors ,cut carpets ,drain down heating system and shift pipes the easy way is to measure the length and height of your old radiator and then get the closest metric radiator that is SMALLER than the old radiator on length. Then all plumbing/do it yourself shops sell radiator pipe extensions which then can be adapted to make up the difference. There are several different types ones which can just pull out to the needed length(this type is unreliable and prone to leaking) others can be bought in lots of different sizes are solid metal and you just buy the one which is exactly the length needed and then fit to the radiator ,another type is solid metal and is cut down to the correct length and finally a 15mm male straight could be added to the radiator and then copper pipe to the correct length and then another fitting depending on the control valves sizes fitted on to the control valve. Plumbers tape (otherwise known as white tape or ptfe tape needs to be added to the threads of the pipe extensions in a good amount . The old and new radiator brackets will almost certainly be different and will need to be changed which would require new fixings to the wall also depending on what your wall is made of. Great care MUST be taken to insure the brackets are solid on the wall because when filled the radiator will be heavy, if the brackets come of the wall the pipes will burst causing SERIOUS damage to the house. You then open both valves check for leaks and AIR the radiator be opening the airing pin (always at the top of radiator) with a airing key until water comes out of the pin (do not remove pin from radiator only open one or two turns as when the water comes out the pin needs closing fast or black water will go over the floor ,walls etc.) If you have a new boiler the pressure gauge on boiler will drop and the boiler pressure will need to be topped up ,normally by a valve which looks like a washing machine valve being turned one quarter turn(however there are other types). The pressure should be between one and one and a half bar no more ,take great care to MAKE SURE you have completely turned of the valve afterwards or the boiler will become over pressurized. Or if it is a traditional system you just open the airing pin and a tank in the loft will fill up by itself.