To produce a comfortable room temperature of around 21º/22º Celsius (70º Fahrenheit), when the outside temperature is 0º, you will need about 1kW of heat for every 14 cubic metres of space. 1kW is the equivalent of approximately 1 bar of an electric fire.
The following formula and example will give you a reasonable guide to the heat output you are likely to need. To establish the cubic capacity of the room, measure the length by the width by the height, for example a room that is in the region of 6.5 metres. (21ft. 6") x 4.5 metres. (15ft.) x 2.4 metres high (8ft.) will require a stove of about 5kW output to adequately heat its 70.2 cubic metres.
A stove with a higher output can be used if it is to be run with a lower fuel level. With a little experimenting you will quickly find the fuel loading that is right for your comfort. A small increase in the heat output may be required depending on the number of windows and doors and the level of insulation there is in the room.