You ask me, well, what about the methods? I can understand what differences in result I want to get without complicated statistical terms about a comparison of the frequencies and correlations. I will tell you: so, the calculation of the sample size is closely related to another statistical indicator — the power of the statistical method that will be used for the analysis. Power is the probability of detecting differences, if they exist, or the ability of the method to do so. The lower it is, the smaller the sample size we need. In a medical research, the power is usually set in the range from 80% to 90%.
Each statistical criterion (that is, data processing method) has its own formula for calculating the power which can be transformed into a formula for calculating the sample size. To make your life a little easier — you don't have to learn these formulas by heart or calculate them manually — there are special calculators. For example, in this calculator statzilla.fvds.ru
you don't have to worry about statistical methods, you just need to select your research question type, the service will do everything for you including the text generation.