Go to Size Standards (U.S. Small Business Administration).
Statistical Abstract of the United States is also available online (the U.S. Census Bureau terminated the collection of data for the Statistical Compendia program effective October 1, 2011). Select the year and then click on Business Enterprise, then the Small Establishments, Employees, and Payroll subsection.
For data on Spain, take a look at INE. Bankruptcy Proceedings Statistics. (data up to the fourth quarter of 2020). Starting in 2021, the Official College of Property and Commercial Registrars of Spain will compile this data. The information can be accessed through the Statistical Registry Portal.