If the data is continuous, then you will need some estimate of the standard deviation. If you know the approximate maximum and minimum values and the data is roughly normally distributed, then you can take them max-min and divide by six as an estimate of the standard deviation.

Other techniques involve taking a preliminary sub-sample (4-10) and using that sample’s standard deviation for the estimate of the larger sample standard deviation. This only works when your intended sample is sufficient larger than your sub-sample. You also need to know what the largest acceptable margin of error is. As the need to have a narrower margin of error increases so will your sample size. –see margin of error.