Apriori sample size calculation for mixed ANOVA

I'm trying to calculate the required sample size for my study to show significant effects. I'm expecting to analyse my data using a series of Mixed ANOVAs (within subject pre-test to post-test change and between group differences for control vs experimental groups). I have at least three different dependent variables to analyse so will have to run multiple anovas and apply bonferroni corrections. My main problem at the moment is I need to run a predictive power analysis to determine the required sample size. I have downloaded G* Power but am unsure of what I need to enter in all the given boxes? (At the moment when I increase the desired effect size it seems to reduce the required number of participants which cannot be correct).
Any help and advice gratefully received.
Many Thanks.