I love the idea that I am Pooh-sized - I've never heard of that before and shall always think of myself that way now
I've rode most of the rides in Disney and have never had an issue - and I am more like a size 26/28. The only thing, as some of the other people here have said, is that some turnstiles can be a bit snug - Country Bear Jamboree and Haunted Mansion (both in MK) are very snug fits! But I still got in. You do learn to perfect going through turnstiles on your tippy-toes
Seriously, I've never been on a Disney ride that was a problem - don't worry about it!
And for the record I have done Tower of Terror, RocknRoller Coaster, Mission Space, Test Track, Soarin, Space Mountain (not very easy to get out of!), Spaceship Earth, Haunted Mansion, Everest, Kali River Rapids (this was a tight fit with me and my fiance sharing a double seat who is also Pooh sized, but we still fit!), Thunder Mountain, Splash Mountain, and loads more.
Hope this sets your mind at rest a bit
Miranda
(Worried I am more heffalump sized than Pooh sized!)