The Small Business Act's mandate to SBA is implemented in SBA's Small Business Size Regulations, which are contained in Title 13 of the Code of Federal Regulations, part 121 (13 CFR §121).

The Electronic Code of Federal Regulations (e-CFR) is maintained by the U.S. National Archives and Records Administration. It includes all changes to the Small Business Size Regulations and is current as of the date specified at the top of the linked page. 

Information about any actions SBA has taken or has proposed to take regarding its Small Business Size Regulations can be found in What's New.