Sample size is derived from confidence level and how representative the sample is, which is the problem you have with surveys (and I mean not just for education but marketing in general). You have to get an unbiased sample, which is tough, typically it’s the disgruntled students who will fill one out or go to rate my professor. You could strip out the extreme responses as a first step, but if your responses are extreme (50% love the prof, 50% hate)…