Here's how you can correctly size a fireplace (Kw) for your room:
Calculate your room volume (m3)
Measure your room width, lenght and height, as illustrated in the
figure below. Multiply those figures with each other to get your room
Multiply the cubic meters by the relevant insulation factor below:
Poorly insulated room: 0.06
Well insulated room: 0.035
*The result, measured in kilowatts, is the minimum heat output required.