The situation didn't cause me much concern as it was abundantly obvious that the property, which several years back had been razed and converted into a 3-plex, WITH PERMITS, had for some reason not been updated in every possible listing. This notwithstanding, the updating of information is important, but, if you have the correct information, all the required paperwork, and permits, etc., it is an administrative concern to be corrected.
In your case, however, it is important for a variety of reason -- less square footage will mean a potential reduction to the fair market value as compared to other properties. On the other hand, if you have this item adjusted now, it could also increase your property taxes, as these are based on the same information.
You'll have to research the county and have them open a case to review the permits, building records, and any other pertinent information to correct this error, (if, in fact, it is).
Did you recently do some renovations that changed the square footage of the property? If that is the case, it is not uncommon to see tax records take some time to be updated accurately and then that information to be disseminated out. If you are concerned that the size is not accurately reflected in the tax record you can check with the county recorders office to make sure that the square footage is accurate in tax records.
If the tax records do indicate the correct square footage it might just take some time for that record to be accurately reflected across sites such at Trulia.
Hope that helps!
Alain Pinel Realtors