using G*Power software, In some circumstances when a researcher does not have good idea about what to expect, he can use Cohens effect size. In the G*Power, if you click on Options you will get the window below that allows you to enter values for Cohens effect size

Click on Use Cohens effect size index h and press OK

Enter the different effect sizes as given below and the corresponding total sample sizes

Cohens effect size	0.2	0.24	0.28	0.32
Total sample size

If the budget for the research is tight, what it the best defensible sample size can you recommend (from the results above)?

Power: ___________ Sample size: ________ Closest effect size: ______