Selection of sample size for clinical study is always critical choice. Few samples do not represent size of target population. On the other hand, more samples than required put more individuals to the risk of the intervention. Additionally, it is wastage of time and resources.
The calculation of an adequate sample size so it is crucial in any clinical study. Optimum number of participants required to be able to arrive at scientifically proven results. Risk index is undoubtedly one of the major factors which are being considered for sample size determination. Though I believe it would not be fair to judge only with one factor. There are many other factors which affect sample size. For example,

• Level of significance
• Underlying event rate in the population
• Standard deviation in the population.

-Hetal