You will need to select the type I error rate and desired power for the test of the intervention effect. The type I error rate is the probability of making a type I error, which is to incorrectly reject the null hypothesis of no intervention effect. Generally, investigators try to limit that probability, so that they do not report chance findings as though they were real intervention effects. Power is the probability of correctly rejecting a false null hypothesis. In general, investigators try to maximize that probability, so that they do not miss a real intervention effect. 

The research community has come to look at 5% as an acceptable type I error rate for a single primary outcome. If there are two primary outcomes, it is common to divide the 5% type I error rate evenly between the two tests, and to use 2.5% for each one. For pilot studies, a more relaxed type I error rate is sometimes used, such as 10%.

Many view 80% as an acceptable level for power. In pilot studies, a more relaxed power of 70% is sometimes used. If the consequences of missing a real intervention effect are substantial, an 85% or 90% level of power is sometimes used. 

The type I error rate and desired power should be selected for the study at hand, in consideration of the relative danger of making a type I error or failing to find a real effect. If a type I error would be serious, that rate should be set low. If failing to find an effect that is real would be serious as well, power should be set high. On the other hand, if one or both of these errors would be troublesome but not serious, the type I error rate could be set higher and the power could be set lower. Setting the type I error rate and power should be done in full recognition that they have a substantial effect on the sample size required for the study.