To calculate the sample size, you need the following parameters.
Z-score: The Z-score value can be found, here.
Standard deviation
Confidence level
To calculate use the sample size, use this formula:
Sample Size = (Z-score)2 * StdDev*(1-StdDev) / (margin of error)2
Consider the confidence level of 90%, standard deviation of .6 and margin of error, +/-4%
((1.64)2 x .6(.6)) / (.04)2
( 2.68x .0.36) / .0016
.9648 / .0016
603
603 respondents are needed and that becomes your sample size.