Agents participate in the MLS and are bound by the requirement that the listing data be accurate. This is an MLS rule enforced by the MLS for the benefit of all MLS users. In this case the MLS listing is closed so the MLS would not take action regarding the error in measurement.
Whether the agent has other liability depends on a host of factors. The standard form sales agreement in New Mexico contains exculpatory language protecting the listing broker from square footage errors. The burden is placed on the buyer to measure the size of the property. This would probably not shield an agent from the natural consequences of intentional conduct.
Whether the buyer broker is liable is always a question of fact and reasonable conduct in your locale. All the facts need to be examined.
Your access to the appraisal would be automatic. Your lender should have it and you can get it.
Bottom line: this is a question of fact that cannot be answered based on your post. A full investigation is required to determine who knew what and when did they know it. These facts are measured against the standards for reasonable broker conduct in your area.
With regard to any recourse, you may wish to obtain legal advice from an attorney, as they are the only ones who can advise on the matter.
Do you know the source used by the listing agent to get the sqft measurement (e.g. floorplan, past appraisal, professionally done, etc.)?
Syan Real Estate
Call/Text: (505) 730-8181