To calculate the maximum size of ethanol fireplace for your room, you first need to figure our how many cubic feet your room is: simply multiply room length by width and by height. Ethanol fireplaces need to have 200 cubic feet per each 1000 BTUs they produce. So, for example, if you have a 1200 cubic feet room, the largest ethanol fireplace for this room would be the one that produces 6000 BTUs (1200/200 x 1000).