Typically you can use 3 measurements....tax role is most common. You could also use builder's plans or appraisal. There are a few appraisers that will measure only for a smaller fee if you think the home is bigger than what is reported on the tax role. Measurement is normally air-conditioned space and does not include garage. If the bonus room is finished you could likely include it in the size, if it is unfinished I would list it seperately....ie 2500sqft home with 500sqft unfinished bonus room.