As a rough guide you need to measure the room in metres:

Height x Width x Depth then divide this by 14.

This will give you an approximate average heating value required for the room in kilowatts, assuming the room temperature is 20 degrees with an outside temperature of 0 degrees.

As this is only a rough guide it can change with the number of doors and windows in the room and other factors such as stairs leading off to other rooms. If you have a well insulated house then you may not need a larger stove and can under-estimate your heating requirement.

Convert feet into metres by dividing by 3.3