It might sound obvious but the basic rule is the larger the room, the bigger the fan required. For a small room (up to 20 square metres) fans with blades up to 122cm are ideal. For a bigger room, you'll probably want to look for a fan with blades that are 130cm or more.
Generally, there are two options when it comes to controlling your fan. Either a wall switch with a dial to vary the speed, or a remote control if access to the roof and wiring is an issue.
Most fans have either three or four blades and are made of either plywood, metal or plastic. The number of blades has no effect on their ability to cool a room but timber and plastic blades do tend to be a little quieter than metal.
Another factor worth considering is how your fan is going to look within a room, as they can become a dominant feature. This is not too much of an issue if it's going to be installed in an outdoor area, but for indoor fans it's worth thinking about how it will complement your colour scheme and décor.
Ceiling fans generally use either AC (alternating current) or DC (direct current) motors. DC models are becoming more and more popular because they are more efficient to run and use less energy than AC models. They also tend to offer a greater level of control, with up to seven speed settings available.