if you are to use purposive sampling in your study, what should you do in terms of a. sample/participant and b.sampling size?

Do you mean what you should write in the research report? If so then state the qualification of the participant, why those qualifications are chosen for this study, what's the supporting argument for choosing those qualifications. Basically state the reasons why you chose the participant. It's the same with sampling size, why do you choose that exact number of participants, what's the basis of it, etc. Every decision in a study should come from sound reasoning.