> library(pwr)
> pwr.t.test(d=0.25,type="paired",power=0.75,sig.level=0.05)

     Paired t test power calculation

              n = 112.9798
              d = 0.25
      sig.level = 0.05
          power = 0.75
    alternative = two.sided

 NOTE: n is number of *pairs*

> pwr.t.test(d=0.5,type="paired",power=0.75,sig.level=0.05)

     Paired t test power calculation

              n = 29.7355
              d = 0.5
      sig.level = 0.05
          power = 0.75
    alternative = two.sided

 NOTE: n is number of *pairs*