Google Earth is an amazing programme, especially its measurement tools. I have compared measuring buildings on Google Earth with an on the ground measurement and the difference is usually less than one metre. In fact using Google Earth can be as good as pacing the building out by foot and a whole lot quicker. It has a number of drawbacks of course, but it is ideal for a quick check of the foot print of a building.
There is a website that uses Google Earth to allow you to easily compare the relative sizes of building footprints and much more. It is called Mapfrappe, click here for link.