In any research study, one of the most important questions asked for at the time of planning the study is ‘what should be the sample size in my study’. Estimation of minimum sample size depends upon- design of the study, whether it is an estimation problem or a hypothesis testing problem, the type of variable(s) to be studied, type-I & II errors acceptable and rough estimates of the parameters to be studied. In this presentation estimation of sample size for a clinical trial, comparing the efficacy of two different drugs, in case of categorical variables and the effect of altering the type I & II errors and the effect size have been discussed with examples. The results showed that the sample size increases when the type-I or / and II errors are decreased. Symmetric nature of sample size on either side with respect to the effect size has also been explained.