That's a pretty meaningless notion: employees, apart from forestry & agriculture, are concentrated in specific (sometimes humongously large) facilities, not spread like butter on a toast. You can look at lease square footage and employment data in annual reports of individual companies.
Boeing Everett, WA, factory. Credit: Map data ©2015 Google Imagery ©2015 , DigitalGlobe, Island County, U.S. Geological Survey, USDA Farm Service Agency
If you want all-encompassing data by industry, there's the Census County Business Patterns (and smaller, like ZIP code) dataset: http://www2.census.gov/econ2013/CBP_CSV/ (available for counties, metro areas, and quasi-ZIP code areas with a two years' lag).
Note: today, the switch to telecommuting employees and outside contractors makes statistics-derived numbers for service industries even more meaningless.