Hello Wile E,
12000 B.T.U's is equal to 1 ton of cooling.
It usually will properly cool 500 sq. ft. with 8 ft. ceilings. If you can install a powered fan to vent the attic it will help, adding 6000 B.T.U.'s cooling to your house. Make sure your window unit is installed level. Also make sure it is properly trimmed, so no outside air can infiltrate into the house.