Unfortunately, there is not a direct way for you to specifically identify small business owners in IPUMS USA data (you can identify those who are self employed using CLASSWKR, but there is no indicator of the size of the business). You may be able to find what you are looking for using IPUMS CPS. Using the ASEC samples, you could use CLASSWKR to identify respondents who are self-employed. PAIDEMP1/PAIDEMP2 and PAINEMP1N/PAIDEMP2N would then help you identify if the self-employed person has paid employees and how how many, respectively. With this, your definition of what a “small business” is would have to depend on the number of paid employees. While imperfect, it may help you conduct your analysis.
You may also want to consider looking into the Survey of Business Owners and Self-Employed Persons (SBO). It looks like microdata is available for 2007 here.