Hi DCR - This is the hardest week of the course! Prepare yourselves. Along with actually having to grapple with some statistical theory (the strange and dreaded Null Hypothesis) and do some math, sample size calculation really forces you to make hard choices about your study design. You will HAVE to chose 1 predictor and 1 outcome variable in order to do a sample size calculation, and this can be painful and feel unrealistic depending on the goals and vision you have for what your study will be. Nevertheless, it really is the only way to get an actual number for your sample size/power. Make the hard choice, do the calculations, and then you can INTERPRET the sample size calculation more grandly when you describe in in prose and argue that the size of the sample that you are settling on is both important (i.e., it will yield results of sufficient precision and sufficient power to reject the null hypothesis) and also feasible.
As noted in the instructions, I highly recommend making at least one of your variables dichotomous for the purpose of making and interpreting your sample size calculation, even if it is not ultimately how you want to analyze your data.
Good luck!
One note - I need to move my Office Hours tomorrow to Monday at 3:30-4:30pm. Sorry about this last minute change - I'll send a separate note out about this.