To achieve a relaxing room temperature of around 21ºC when the external air temperature is at freezing (0ºC) you will need approximately 1kW of heat output for every 14 cubic meters of space.
Measure the length, width and height of your room and multiply the three figures together.
For example, a room measuring 7m long by 4m wide and with a height of 2.5m is 70 cu. m. of space. Divide by the sum by 14 and this means you will require a 5kW stove.
However this is just a rough guide, factors such as the number of outside walls, the size of windows and whether they are double glazed, the age of the home etc, can all influence the heat requirement.