Hi, I think you can try with this link.

https://www-users.york.ac.uk/~mb55/msc/trials/sampsz.htm

An another link...

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC137461/

NOMOGRAM: This method of sample size calculation may be obtained through the formula below:

n = (2/d2)Cp.power

... where n = sample size; d = standard difference; and C = constant read by pValue and % power.

---------------------------------------------

                      POWER

p          50     80     90        95

0.05    3.8    7.9    10.5    13.0

0.01    6.6    11.7   14.7    17.8

----------------------------------------------

See attached excel for illustration.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC137461/

Thanx all

To do power analysis to estimate your sample size, you have to write your hypothesis, and based on that you decide what statistical test you will use. It should be one of the inferential statistics. so you need to determine the following: alpha {standard to be .05}, power [standard to be .80], effect size {small, moderate, or large, each test has its own value, you can find these values in the net}. Then download free programs to calculate the sample size such as G. power.