Determining sample size for patient outcomes depends on what sort of outcomes you are measuring (e.g., continuous, binary, polychotomous, ordinal) and whether those outcomes will be considered together (multivariate) or separately (univariate). Next you need, among other things, an estimate of the expected effect size and, if outcomes are continuous, estimates of variability. Once you have this information you need software that will estimate the power and sample size. A power and sample size estimation can be quite complicated and is best done with a statistical consultant.