Hi J Arthur,
The SHORT answer to your question is "NO", there is no standard way to measure the square footage of a home. If and when your home is listed in the Multiple Listing Service, there is a section regarding Living Area. In this spot, the listing agent will be required to note the "source" of the living area ("Appraiser", "Measured", "Other", "Owner" or "Public Record").
An appraiser will not count living area (finished or not) in the basement or attic, whereas the town assessor (Public Record) might count these areas. It is all objective. This is why your listing agent will most likely disclose in your listing that "Living area is approximate and should be confirmed by the prospective buyer:")
I hope this gives you SOME clarification!