The ‘a’ standard extract files have an item called ‘landsqmi’ that is the land area of the census tract — water areas have been subtracted out. These files, and the documentation supporting them, can be found by county (stored in directories by state) at http://sedac.ciesin.columbia.edu/data/set/acrp-standard-extract-1990.

The files are all named with a ‘tr’ prefix, e.g., tr0380a.zip would have the data for county 80 in Alaska.