I am currently preparing my research for ethics submission but trying to establish an accurate sample size using G*Power.

Apologies in advance if I do not explain this accurately and will try to simplify it as much as possible.

I have three groups that will be determined based on test scores. Let's say G1, G2, and G3.

In the experiment I am going to be comparing each group on several different measures (6 and 1 confounding variable).

Will this require a MANOVA?

As one of the tests have small effect sizes, I am concerned I am going to need to recruit an unachievable sample size. However, I have read where a MANOVA analysis is concerned the effects sizes should be adjusted. Is this correct?

Thank you in advance.