Which counties have sample sizes large enough for accurate aggregation of CPS microdata?

I am working on a research project where I need to determine county-level union density in a single year. Because the sample size for particular counties in a single year are so low, I am pooling three years’ worth of data on union membership to increase the number of observations. Nevertheless, I know that certain counties have too small of a sample size to have accurate estimates at the county level.

Does IPUMS or CPS provide a list of counties whose microdata are unreliable for aggregating to the county level because the sampling is incomprehensive or too small?