Few questions for you:
How short and fluffy are we talking here?
Is there a Disney character that you love? Or an attraction that you love? Think out of the box: For instance, I adore the Haunted Mansion girl, so that's my costume usually. There was a poster here who costumed as Space Mountain last year. My favorite costume that I saw last year was a couple who were dressed as Anita and Rodger from 101 Dalmatians. They looked great!
How about your grandson? Who does he like? Our son LOVEs Jake and the Neverland pirates. Last year we dressed him up like a lost boy from Peter Pan, but I think we will all be Jake characters this year.
Can you make a costume? You don't have be be super-crafty - check out Disneybounding - there are lots of neat ideas on how to make costumes from regular clothes. Or check out your local Micheal's craft store since they have sewing classes - especially around Halloween.
While it does help to play to your body type, sometimes, doing a costume that you love is the best! So what if you want to dress as Malificent, but you lack a few inches? Make a rockin' costume and everyone will forget that you are a little short for it. Do what you want and do it well!! And check out Disneybounding.
Let us know what you decide - good luck and have fun!!