Keep in mind that, when choosing a room air conditioner, size is important. A unit that is too small will run continually without cooling the room effectively. On the other hand, if you buy a unit that's too large for the space you're cooling, it actually will be less effective than one that's the correct size. Bigger isn't necessarily better.
Air conditioners work by removing both heat and humidity. Humidity condenses from the air when it passes over the air conditioner's cooling coils. So a unit that is too big will cool the room so quickly that it won't have the opportunity to remove as much humidity from the air as it should. As a result, the unit will keep turning off and on and the room will feel damp, clammy and less comfortable. A properly sized unit, on the other hand, will remove humidity effectively as it cools.
Also remember that larger units cost more and use additional energy. Running a smaller unit for a longer period of time will use less energy to completely cool a room than running a larger unit for a shorter time.