You need to find out the temperature that you wish to maintain inside the room to be comfortable. Then that temperature minus the ambient temperature will be the amount of temperature raise, you need to increase by heating. Now the room can be big or small, hence the volume of the air inside to be calculated. Further the room can be empty or full with objects( Some objects hold the heat for a long time, some may also require more heating to attain the required temperature) , Having done all this you need to consider the loss because of conduction through the walls, radiation and convection...... Too much is it ??? Try from the smallest available with your electrician and keep increasing till you are comfortable