Once you have the measurements to the first floor, if it's a rectangle, and fairly standard, then the second floor should be the same? Otherwise, you'll have to do the same on the second floor. Or it might be easier to measure how much of a 'cutout' there would be from the first floor measurement.
Also, in syd's directions, make sure you measure any front or back porch or extension separately, then add it to the main building.
If it's two bedroom it's probably not more than 100m2. Unless it's got really big rooms.