Power sampling

Hello,

after a unsuccessful search in lists maliarchive I wonder how I could 
estimate the power of a sample size related to an unknown population.

Given the following (fake))situation:

I do have a database containing about 5 millions observations over 70 
variables.
I would like to compute (as epidemiologists are used to) the required 
  size of a sample to do some test on a test sample (test data), later 
doing some subsequent analysis of a new sample to build a prediction 
model.

Help facilities of R show some entries regarding power, but none of 
them seemed to be appropriate for my purpose (maybe I am wrong, but 
sometimes I have some difficulties to decipher the message of those 
tiny hints for available packages)

I would appreciate somebody effort to point me to the right 
package/function to archieve this task!


regards

Thomas