Sample size calculation for cohort study

If it's a feasibility study do you really need a power calculation? You might, but the purpose of a feasibility study is to assess the feasibility of the methods, not to establish some statistical difference (although you can use the numbers you get from the pilot in the power calculation for the full study). Hard to say without more details. Just don't try to generalise the results beyond your planned main study.

Here's an article about sample size justification, which is not the same as a power calculation.
Thank you for your response.
We also want to perform stat analysis so wanted to ensure sample size was sufficient for analysis.
But what does 'sufficient for analysis' mean in this context? Are you really trying to establish the efficacy of some treatment? Or are you trying to see if the study is feasible? You shouldn't get a p-value just for the sake of having one. What would you want to show in your analysis?