The first thing that really should be said about small window air conditioners is that not all of them are only going to be useful for small areas. Just because the unit itself is smaller in size does not mean that it might not contain enough power to cool a larger area of the home. You can not always just go by the physical size of the unit in order to determine what size of a room it is going to cool. This is actually done by looking at the BTUs of the unit, which is going to show you how much power the unit has and its ability to cool a room.
One of the most important things that people look for whatever choosing from among the small window air conditioners that are available is the amount of noise that it is going to make. This is especially important if the AC unit is installed in the bedroom window where noise is often a consideration that can affect your quality of sleep. Another thing that may need to be considered is the style of the unit, particularly if it is installed in more of a public area of the home.
Some additional features that you may want to consider are that many of these small window air conditioners can also act as a dehumidifier or even as a fan, without the cooling unit turned on. Something else that many people find convenient is if the unit has a digital thermometer so that you are able to control the temperature inside of the room a little easier. One final option that you might want to consider is getting a unit with a remote control, you would be surprised with how many times you will use it. Although these additional options are not necessary, they do help to make the unit a bit more convenient.