What about the R package pwr? Not sure about stratified samples, but you can easily compute the power, or alternatively estimate the parameters required to reach some particular power. Example from the documentation:

pwr.p.test(h = 0.2, n = 60, sig.level = 0.05, alternative = "two.sided")

 proportion power calculation for binomial distribution (arcsine transformation) 

              h = 0.2
              n = 60
      sig.level = 0.05
          power = 0.3408451
    alternative = two.sided

If we want to know for the above how many samples we need to achieve 0.8 power:

pwr.p.test(h = 0.2, power = 0.8, sig.level = 0.05, alternative = "two.sided")

     proportion power calculation for binomial distribution (arcsine transformation) 

              h = 0.2
              n = 196.2215
      sig.level = 0.05
          power = 0.8
    alternative = two.sided

Almost 200 samples! Also take a look at the plotting functions.