The calculation of the energy demand for the room to be suplied depends on two major factors: Type of use and quality of insulation/airtightness.
This might not be of much help,sorry.
But you should try to put all the radiators always into the coldest spot of the room , that's usually under the windows. If you have two windows in the room go for two smaller radiators to be placed there instead of one large one under one window. We had a thread here on AAM/homes and garden before, it's worth checking through. You could go to a radiator supplier and tell him a.) the size of the particular room and b.) the state of heat retention, at least the time/period of building the house. There isn't much price difference between a smaller and a one size larger radiator , as little as 10 or 20 €'s. The larger you choose the radiator the lower the run temperatur (boiler) can be set, making it more economical and ecological.
If space (under the windows) is a problem then go for double or triple shelled radiators , they would have a higher output than a single or double shell. Keep the minimum distance to the wall and floor , if possible a bit extra to increase air fluctuation.Most wall fixings coming with the radiator allow for two or three different spacings, go for the widest.If necessarry you could still decide for a smaller distance, the radiators are just hung into the fixings. If renovating an old building it makes very much sense to put an extra layer of insulation just behind the radiator(if available with a heat deflector foil which costs little, 2-3€ per radiator) glued to the wall. This could be a 1cm or 2cm strong sheet of aeroboard with an aluminia foil on top. Fixed with wall paper glue, read the label on the glue packet if suitable for aeroboard. Most brands are so.
If you have to finish the last floor board before you know what distances are necessarry for the pipes then flexible heat and pressure resistant hoses are available from B&Q and from the various plumber shops. But you could still finish the floor minus the last board and leave a lenght of 1.5 meters of pipe sticking out untill you have decided for the radiator.
Don't forget to purchase the thermostatic radiator valves!