Part of deciding on the size of an experiment is literature search. Has the suggested response variable been used before? (Most possibly.) What rate of an event is to be expected? What distribution does the response have? More often than not, however, especially the last question cannot be reliably answered, as many articles report effect size, p-value, period. If the hapless statistician is lucky, he will see the desired data in a box-and-whiskers plot, a dot chart or a scatter plot. A couple of times I have found myself trying to recover the original data using a ruler.
Ceterum censeo: Raw data now!