To determine the 'appropriate' sample size, you need to set the level of Power you wish to achieve. (ie., the probability your study will find an effect) Usually, this is set at .80 or 80%. If you download the program G*Power, it will compute the necessary sample size for you automatically. You can download it for free here: http://www.psycho.uni-duesseldorf.de...n/aap/gpower3/

It will allow you to calculate sample size for a variety of tests (F, t, z, r)

If you are wishing to compare correlations between groups, you can have a look at the following website. (Scroll down to section 4)

http://www2.hawaii.edu/~donaldp/Bios.../lecture23.htm