Other than the appraisal, which if you don't have a copy, perhaps your mortgage person that you worked with retained a copy?
You might be able to get the information through the city's online building permit services, but from what I've researched just now for my own condo building, it looks as though it may not be available. Here's the link: http://services.sfgov.org/dbipts/
However, one other possibility would be to venture (or perhaps I should say..."adventure") down to the city's Department of Building and Inspections. As a homeowner, you can request all information filed for the building of your property. The architectural plans should have the square footage of the building noted. If you can retrieve those documents from the DBI.
I hope that helps. -- Kevin