HELP: How to calculate sample size for prospective cohort study

I'm trying to calculate the sample size for a prospective cohort study in which I will be comparing risk factors in 2 groups: group 1 will have a psychiatric illness, group 2 will not (my controls). This study does not include any time of intervention, but instead I am following them over a period of 8 months and assessing various predictors (i.e. depression and anxiety severity). I will also be using a questionnaire as my primary outcome (looking at total score).

How would I go about calculating sample size for this type of study?