is your house for sale? if not, then why does it matter?
trulia isn't a real estate company, it pulls data from multiple sources and aggregates it here. if your house is not currently listed, then the sq ft info probably comes from the county tax record database. you most effective way to correct is to go to the county and have it corrected there.
if your house is listed, then talk to you agent about making the correction.
if you add 1,000 sq ft to your county records, your property taxes will increase because it's based on sq ft.