How big of house will a 3.5 air conditioner cool?

A 3.5kW Air Conditioner is probably only sufficient to cool one room. It should be okay for something like a bedroom or small living room, depending on the ambient temperature. If you're looking for 5 degrees C of temperature drop in a room bigger than 12' square with an 8' ceiling then I would go for something bigger, like a 5.7 kW or two 3.5kW units.