Stata 13 has come with a new tool to aid in Power and Sample size calculations. The abilities of the new feature includes:

• Power and sample size calculations with a graph showing you the relationship between power and sample size. and
• a sensitivity analysis for your sample size calculations to find an appropriate balance between the desired detectable effect, sample size, available resources, and an objective of the study.
  

The stata PSS manual http://www.stata.com/manuals13/pss.pdf gives more details.