Determine whether an oversized air-conditioning unit is to blame for uncomfortable conditions in your home:
1. Determine the nominal size from the model number. The last three digits on the outdoor unit show the nominal capacity in thousands of BTU/hr.
2. Use the manufacturer’s data. Look for both the model number of the outdoor unit and the model number of the indoor coil.
3. Use the AHRI Directory. The data here usually matches the manufacturer data. Enter your information a bit at a time for best results.
4. Use Preston’s Guide. For an older unit, you’ll need to know the manufacturer, the outdoor unit model number, and the type of your system.
Read the full article here: Four Ways to Find the Size of Your Air Conditioner | GreenBuildingAdvisor.com