Vampire, whoever is going as that parrot is my new best friend!!!
As for how to measure for something like that, the best thing that I can think of is to simply measure the height of the person and plan for the belly to be rather large while creating a "ribcage" for it. I've worn school mascot outfits before, well, one anyways; it was a penguin, and that's kind of how that one works. Oh, the wingspan should be considered, too. Beyond that, no clue.
