In the course of doing research, one difficult decision to make is knowing which statistical test to use and at the start of the research, what is the sample size required.
There are several internet resources for those who are not trained as statisticians and for those without statistical sofware like SPSS or Stata.
One nice website is the Decision Tree for Statistics. It helps in selecting which statistical test are appropriate for the purpose or condition of a particular study or analysis.
I will also recommend this page which collects links to webpages that perform statistical calculations at statpages.org.
Here is also a link to one sample size calculator for medical related studies.
Hope these links will help you.