hi,

in power calculations you have 3 variables (power, effect, sample size) and by fixing 2 you can calculate the third. Of course you always have to have an idea of the population standard deviation.

So, what is asked from you is to use the power calculation program and input the sample and the power (80% I presume) to see what is the smallest effect size you can hope to detect (provided you estimate the standard deviation well enough)

regards