Is my population large enough to be meaningful (not sample size)? I have a project where I have access to entire populations' records. I'm not trying to generalize knowledge to a larger population; rather, I'm trying to compare the prevalence of an event happening in one whole population to the next (think comparing what percent of shoppers buy a certain product at Walmart versus Target, where you have records on every single shopper at both).
But, to continue using this example, some stores are very small--maybe only 5 customers in a month, as opposed to 500 at another.
I want to figure out what my minimum population size should be to include in the comparison. I have to keep stores separate and not lump them together. I've tried to use sample size power estimations, but it doesn't seem like the right fit for the dilemma. But perhaps I'm wrong.
Any advice?
 A: From questions and comments, it seems that you don't want to do inference but you want to generalize your conclusions. However, inference is just generalizing. Then, either you don't need to generalize because you have data on the whole population, or what seems the entire population is a sample from the population you are actually interested in.
If you are interested in comparing the percent of customers in every store that bought a given product, with the whole population it's easy to compute both percents and tell the larger one. For example: "10 out of 1000 (1%) customers of Walmart have bought the product, while 1 out of 102 (0.98%) customers of Target have bought it; therefore, in that period Walmart performed better at selling the product".
However, we are usually more interested in knowing which store have been more effective, if there is a significant difference. Here we are interested in the probability of a customer buying the product, not just on the proportion of customers having bought it, and now our population is the pool of all possible customers that could have visited the stores, and the customers actually visiting them are just a sample of that population - the "hypothetical larger population" in your comment.
