Salty,
In a perfect world, you could get away with an 8,000 btu unit for 192 sq. ft., but since it's (i guess) a metal storage unit I would go with 10,000 btu. I would go with Haier (but thats all I have direct experience with). Make sure, what ever the brand, it uses the self-evaporation technology.
Here's a couple links
http://www.haieramerica.com/en/product/CPR10XC6
Good luck