There is a lot to think about when you are considering a new air conditioner or heater. However, whether it is a ducted reverse cycle system, split system, or a new ducted gas heater, you always need to think about the capacity of the unit you need. It can be a puzzle if you don't know what factors to consider. An installer can generally help you with this too.
Ultimately, selecting the right air conditioner depends on numerous factors including room size, ceiling height, does your home have insulation, is there a big window in a room, and does it receive a lot of sunlight?
How is capacity measured?
Generally, reverse cycle systems (ducted or split systems) and gas heaters are measured in kW output capacity. The bigger the kW number, the greater the capacity of the unit.
With a wall split system you need to consider the size of the room that it is being installed in, but with a ducted reverse cycle or gas heater, you should consider the area of the entire house.
Don't just get the most powerful unit, you may not need it. In fact, if you get a unit that is too powerful, it'll constantly be turning turning itself off and on again as it would be sending too much conditioned air into the room/s and then shutting down. This can increase wear on the unit, and isn't very energy efficient. On the other hand, a unit that is too small or not powerful enough for the room you are in will not be able to make you comfortable.