If the room is well insulated and not on the NorthWest corner of the house I would think a 50,000 btu unit would work well. If the qualifications are not there it may take more like 80 to 100,000 btu. Like InspectorD said the dealers will be happy to size it for you. For economical operation it is best to stay on the low side, whereas the unit would run full time on the worse days and might even loose 2 to 5 degrees but would cycle on and off on milder days. Of course, your location probably gets more extreme cold days than I would ever imagine. Your best shot is, talk to the dealers.