Simone,
Without some estimate of the effect size and the desired power, there is no principled way of determining the needed effect size.
I would look at related studies in the literature, get an estimate of the effect size there, and use that effect size in a power calculator (like G*power http://www.psycho.uni-duesseldorf.de/aap/projects/gpower/).
Assuming two independent groups (and the use of a Cohen’s d -like effect size), you’ll select the following options: Test Family – t tests; Statistical test – Means: Difference between two independent means (two groups); Power analysis – A priori: Compute required sample size given alpha, power, and effect size.
Using 30 subjects, you can detect a Cohen’s d of approximately 0.75 (with power of 0.80, alpha of 0.05 and an N2/N1 ratio of 1). If you think the effect will be smaller than that, you’re study will be underpowered.
Hope that helps.
Rafael