I would ask the seller if they know the Square Footage of the house, they may have survey done previously. Another way is to get the measurement of the rooms and calculate them. Also to get the genral idea is to maesure the house..width+length=total Square feet...if it's only 1 level house.
I would mention to buyer atty and make an appointment with the seller, ask seller agent also if they can find out the details. You can check with Brookhaven Town 631-451-6300 and ask how to go about finding out the Square Feet of this house, if they have any record, you may be able to get the answer over the phone.....Good luck..