That's a tricky one, especially for Little Mermaid since most of us and the audience know what Ariel looks likes from the movie. Not that you should let that drive your casting decision. Yes, a professional theater might not cast someone who looks too different, but I don't know if you need to go over that with the students who are auditioning - it might make some girls more self-conscious. This is something that I've brought up occasionally in my Advanced Drama cast, but not with students auditioning for a play (in part because I haven't had a situation in which appearance was that important).
I guess the first thing to decide is if you actually would cast someone whose body type is different from the standard Ariel. If yes, make it clear you are casting on talent and who can handle the role best, not who looks like Ariel the most (that's what costume and make up are for). Also, perhaps before auditions you go over the planned costume with the girls, so they will have an idea what they will have to wear.
Don't bring it up at all. Cast the best girl for the role and be done with it. Let anyone audition who wants to they gain experience that way as well. Actors can fit many parts given opportunity to rise to it. I tell all my actors that it is a numbers game for parts just keep auditioning and working on your skills and one day it may be your number that gets the part.
Break a leg and may all your theatre seats be filled,
Kelly M. Thomas
Department of Theatre
Dr. Ralph H. Poteet High School
3300 Poteet Drive
Mesquite, Texas 75150