There maybe different ways to find out...

First,you try to go online on your county accessor's office. In my state (NV), we can find real property records open to public online, usually using the parcel No. Because propery tax is based on the size of the house and lot and improvement.You can even find out all the previous owner info.
Second, in MLS system, what's the size of the house?
Third, when you do title search, the title company will have the precize info of this house.
Fourth, is this a custom made home? If not, you may find out the builder info and neighborhood home layout and size.
All these above, will help you to decide if the seller is lying or not.
If you really don't want this house anymore. The only way to get out is law suit. It's going to be ugly. find a good lawer, sue the seller, seller's agent, your agent, morgage company, title company, every one invloved in this transaction, because they all responsible to certain degree. Somebody will back out eventually.
You are probably buying the most expensive commodity in your life time. Just be careful!