I am exerting my efforts to identify the impacts of last year's three-month prolonged flood on the mental health of the farming communities of Pakistan, particularly in rural Sindh province. Therefore, I have two options in hand to choose either: first, I should go ahead with a focus-group discussion (how long should be the sample size?); second, I can conduct a survey questionnaire (how many questionnaires should be filled sufficiently).
Also, how to develop this kind of questionnaire, as I do not find any relevant study that could be sufficiently helpful to follow?