Finding the correct AC size (or other AC unit size) is a priority to making your home comfortable. An air conditioner is an investment, so finding the right one the first time can make the purchase process go much more smoothly. it is very important if you want to minimize your electricity bill. A small air conditioner will have a hard time cooling a large room, or it might not be able to cool it at all.
On the other hand, choosing a huge air conditioner could cool the room very fast, but won't dehumidify the room at all. This particular instance leaves the room cold and, at the same time, clammy, which is not comfortable at all. Therefore, it is crucial to decide what size of air conditioner you need before buying one.