Hairspray Jr. diversity and other elements

I am interested in doing Hairspray, but have some question. Does Tracy have to be a girl of larger size, can you get away with someone who is on the more average size? Do I enhance her size through costume? I do have some diversity in my school, but how many students would I need to represent the show in the best way possible? I can cover the principal parts, but I am concerned about the supporting cast and ensemble. What modifications can you make without losing integrity.