Use the One-Sample Standard Deviation option on the Sample Size and Power window (Sample Size and Power Choices) to determine the sample size needed for detecting a change in the standard deviation of your data. The usual purpose of this option is to compute a large enough sample size to guarantee that the risk of a type II error, β, is small. (This is the probability of failing to reject the null hypothesis when it is false.)

can select Larger or Smaller from the menu to indicate the direction of the change you want to detect.