Did you get an answer from Disney on length? My DD is Elsa this year and I am wondering what I need to do the cape that came attached to the costume.
I am a rule follower...better safe than sorry. That said, at last Sunday's party I definitely saw adults in floor length capes and a kid in a full face mask.
Surely the cape thing is also partially that a waist length cape is a less likely to get stuck in ride mechanisms, snagged during rides, etc, etc. I'm going overboard to make sure my costume isn't over the top but still recognizable and falls within the rules.