You would need to prove they did with teh intent to harm, if you purchased it and proved it you may have a case, but if you only are looking at it, you have not been harmed yet. Unfortanately most mistakes in size are either a lazy agent using previous calculations from another source that were not checked or actually measured by themselves or as honest as a miscalculation.
Square footage is a basic math calculatio by taking the dimensions and multiplying. If soemone is adding square footage that is not there, they are not helping anyone in the long run. You could always wrote a letter of complaint to teh managing broker of teh firm, however as far as illegal, you have tio prove it was deliberate and unless tehy confess, it is a tough chore.