Audiocratz explained....choosing right size Studio Monitor...... Reason.... it's hard to gauge which Size you should buy according to your room size. ..... So what exactly is the difference between 5" inch, 6" inch and 8" inch studio monitors? ...Well to explain it simply it is the size of the large circular speaker cone area usually located at the front of the speaker.Larger the cone area it produces more volume, more frequency range and better accuracy..... in the audio output. The size of the room you are working in should also play a very vital role in your decision making process.  All rooms have acoustic problems.  Your speakers are essentially introducing energy into the room and pumping audio through them causes a build up of sound pressure in the corners.... and standing waves in the listening position.... which will either cancel or reinforce certain frequecies in your mix such as the bass.........Since you will hear exaggerated lows, you’re likely to compensate for them, and when the mix..... is played elsewhere, it will lack low end. This effect is much more pronounced in Small rooms than the...Larger rooms... because the walls are closer together and so the reflections are stronger. Therefore If you have a small room then choose studio monitors with a smaller driver to ensure you are not exacerbating the problem....... If you work in a medium to large room or a professional recording studio then 8" monitors would be a better fit because they can move enough air to accurately reproduce frequencies at the lower end without the constraints associated with a smaller enviroment. It's all about air displacement at the end of the day and you would want to move as little air about the room as possible, you need to tailor your choice of studio monitors to your enviroment. You can then invest in acoustic treatment to better control the acoutics problems that exist in all rooms.