What are the differences between the speakers Gear4DJs sells?

Generally, the higher you move up the speaker range the more you get in three important aspects of sound - power (which relates to the volume you hear), frequency range (typically larger speakers will be able to produce lower bass frequencies better) and sound quality and clarity. For example the Citronic CV8 is a great value speaker at the bottom of the range which has a driver size of 8 inches, and a power rating of 75W RMS. For its price this is fantastic, but when compared with an FBT ProMaxX 14A, which has a driver size of 14 inches and a power rating of 900W RMS it's easy to see (and hear) the differences.

What power rating should I look for?

Typically the best measure of continuous output is the RMS (root-mean-squared) output. Compared to other measurements such as Maximum or Peak raying, RMS calculates an average of power that a speaker or amplifier can output, while other ratings such as Maximum or Peak can be deceptively high.
How much power do you need to fill a room?

We recommend the following: 

             Room Size         Up To 100 People         Between 100 and 300 People  Between 300 and 500 People
          System Power                            500w                        900w                      1.2kW