For what my 2˘ is worth...
If little bit is good, a lot must be a lot better isn't
true regarding Air Conditioning
You can actually overdo it by having too large a AC. If you use a air conditioner that's too big, it'll start and stop too much,(called short-cycling) that not only wastes energy, it wears out the AC, and most importantly, it won't be able to dehumidify the room air, making things less comfortable as well. It'll feel like it's NEVER getting cooler.
Figure first the square footage of the area you intend to cool, to determine the size of AC you need...figure the length, times the width to get the square feet.
Then determine the BTU's using a rule of thumb
Room size: 100-250 square feet, Air conditioner size: 5000-6000 BTU
Room size: 250-400 square feet, Air conditioner size: 6000-8500 BTU
Room size: 400-550 square feet, Air conditioner size: 8000-11000 BTU
Room size: 550-875 square feet, Air conditioner size: 11000-15000 BTU
For the record...13' Scamps (like mine) need a AC of only 5000 BTU
Ask.com has a good article on the subject.