Many people choose a stove based on its physical size, to match an existing hearth or opening. This has little relationship to the heat requirement for the room.
Not withstanding variations in heat loss in individual properties. As a general rule you will need 1KW of heat for every cubic metre. This is based on a room temperature of 21 degrees Celsius at an ambient of -1 degree Celsius.
Or you can determine the output required into a room by measuring itīs size and calculating with a simple sum:
HxLxW (metres) DIVIDE BY 14 = Output to room.
This does not take into account any stairs in the room, draughty windows, open doors, arches etc which may mean you need to increase the stoves output to compensate for heat loss.
Function: Do you want a boiler to run radiators and hot water or just a room heater to heat one area?