to figure size of heater needed..multiply square footage by ceilg height..(cubic feet now) and then multiply that by 5.5 to 6.0 for the BTU size heater needed..example.600 sq foot room with 8 foot ceiling would be 4800 cubic feet by 5.5 is 26,400 so at the low end or minimal size heater a 27000 btu heater would heat it..4800 times 6 is 28,800 so a 28,800 btu heater at the max ( for efficiency) would work very well. too small will not heat a room well enough...too big will use more fuel than needed to do the job so try to stay close