I apologize for not finding and answering your question sooner.
I'm afraid that the answer below is not entirely correct. Only listings that have been submitted to Trulia manually can be edited at this time. It looks like we receive your listing information from Houlihan Lawrence, in which case you'll need to contact them to ensure that they include the updated square-footage information for your listing on their site and within the data feed they send to us.
Please let us know if you have any further questions about this.
Nice to see you here. We have talked on the phone.
Go to My Trulia in the top right of your screen, then click on My Listings.
If you have yet to claim the listing, do that first then you can edit it. Be sure to check the MLS as that is where it is pulling from to begin with.