The SBA defines a small business concern as one that is independently owned and operated, is organized for profit, and is not dominant in its field. Depending on the industry, size standard eligibility is based on the average number of employees for the preceding twelve months or on sales volume averaged over a three-year period.
Here are SBA's Table of Small Business Size Standards based on NAICS 2002. HTML PDF
The U.S. Bureau of the Census offers a crosswalk table on its NAICS web site for cross reference tables for the following size standard tables: NAICS 2002 to NAICS 1997, NAICS 1997 to NAICS 2002, and NAICS 2002 to SIC 1997.
SBA provides a search tool to locate NAICS 2002 codes and their size standards, as well as their original SIC codes.
Following are the most frequently used definitions of small business classifications. These definitions are more formally defined in the Federal Acquisition Regulations (FAR). Further details and information about how to qualify for any of these classifications is available through the Small Business Administration (SBA) website. This is not a complete list.
If you qualify for one or more of these small business classifications, and you wish to be listed as an interested small business for federal government bid packages, it is possible register on the Department of Defense Central Contractor Registry (CCR).