Are you looking for sq. ft. for land parcels or the sq. ft. for the buildings on them?
The County Appraiser's site might be a good place to start. You can pull land sizes and building sizes from their web site. However, you will still need to identify which properties are commercial. Frankly speaking there are a great many parcels that would fall into one category or another of commercial property. The task you describe is likely to require considerable time.
Dec 29, 2010