A 115 volt, 10,000 BTU Window Air Conditioning unit is capable of cooling a 500 square foot room without much strain on the unit. So a good rule of thumb is 20 BTUs per square foot.
So for example, if your room to be cooled is approximately 10 x 25 you would have a 250 square foot space to cool, and would require a 5,000 BTU unit to handle the job.
Caution when choosing a unit not to "way overdo" the BTU requirement. Many will buy a much larger unit thinking they are doing themselves a favor. To much cooling can lead to environmental hazards from moisture build up (such as mold), and place an unnecessary draw on your utility bill.
Whenever purchasing an appliance, be sure to review the EnergyStar rating on the label if the unit is EnergyStar rated. This yellow label will estimate the annual cost of operating your unit based on the now 2007 National Average for Electricity Consumption that's recently been published.
Good luck selecting your a/c unit.