in short:

The design would be a placebo-controlled randomized controlled trial where one treatment arm would receive this profylactic treatment and one would receive a placebo.

My outcome is dichotomous: the occurence of a post-operative complication yes/no.

From what i've read I think i would need to calculate my sample size using the Chi-squared test, but i'm not a 100% sure, since i might also apply multiple logistisc regression for analysis of the results.

One of the methods I tried was trying G-power, in which i chose the Chi-squared test + goodness of-fit tests a priori: I used a power of 80% and an alpha of 0.05. what i'm left with is the Df(degrees of freedom??) and effect size W. The biggest problem is that i have no clue wat to use for effect size and Df.

Another method was using the browser based calculator clinalc, in which i only had to fill out the anticipated proportions in the treatment and placebo group in combination with power, alpha and an enrollment ratio(=1). The problem here is that this results in huge sample sizes which would be unfeasible.

Hopefully somebody can help me out!

If any additional information is required i'll try to awnser.