Sizing a sauna room – The general rule of thumb is that you will require 1 Kw for every 50 cubic feet of sauna room. So, as an example for a 5.5' x 5' sauna room with a 7' ceiling you will require 5.5 x 5 x 7 = 192.5 cubic feet so you will require a 4 Kw or bigger sauna heater.