The following is a link to an article on Severe Acute Malnutrition (SAM). It provides clear details on how to determine sample size for a case control study:

http://fex.ennonline.net/45/determining

The following is a link to a tutorial series on RCTs. This tutorial illustrates the derivation (and formula) for estimation of required sample size for a Clinical Trial with a normally distributed outcome:

http://www.scopeonline.co.uk/pages/tutorials/samplesize3.shtml

I will discuss these resources on request.