Seems like tall + skinny would be the standard size for most dancer leotards. But, what do I know?
I looked real quick at google for what Princess Tutu looks like, and.. well, I found several tutorials
on making the tulle fullness portion of a tutu. You could probably download this pattern
for the bodice. Fudge it on the skirting that goes on top of the tulle.. Decorate and embellish to taste. But if it's for Halloween you better hurry!