Most use the open star tip. For normal size cupcakes a lot of people use the 1M tip. Not sure if you would use smaller open star tip for the smaller cupcakes. I am sure someone here who is more familiar will be able to tell you.
I would say try a smaller tipped star (sorry can say exactly what size it is) why not do a trial with a cupcake first or on parchment paper to see what you personally like best. Sorry I could be of more help