Just thought I'd pass along this tip for Dallas County residents who'd like an easy means of determining the square footage of their property. The Dallas Central Appraisal District's web site allows you to find your property on a map. One of the features of this map is to be able to measure area by way of clicking points which define a given area. Simply zoom into the map until you find your property, then at the top of the map choose "Measure" and define the area to be measured. The map will update with each click. Pretty sweet. :8
You'll still need to subtract the area that your house, driveway or other structures cover for determining the square footage of your yard.
DCAD Property Map
Other counties might have similar services as well. Hope this is helpful.