Stomach and butt will be fine

My good friend is Pooh sized as well and she carries most of it in her butt and thigh area...she fits well also. Screamin's bar is over the shoulders. Most of the other rides are lap bars. Neither one of us have ever had a problem. My biggest issue is getting up from space mountain, but it's never hindered me (or her and her hips are bad) from riding. It's so stressful being Pooh sized and not knowing if you will fit (and it's embarrassing if I don't). We went to Universal in Orlando in October and I was very limited in what I could ride. Just this last weekend we went to Great America and I can't do any of the coasters there...that's why I love Disney! One size fits all!
Try not to stress and don't worry about not fitting! You will have a WONDERFUL time!