Bambi,
This is a very common concern, and I can tell you very happily that Disney is very accommodating. I believe that you will find all of the rides and attractions open for you to enjoy.
There has been a great book written on this subject, Passporter's Open Mouse, and it addresses Disney World for special needs, including pregnancy, medical restrictions, physical disabilities, and size restrictions (or as they call it, 'pooh-sized travelers). Also, if you visit
www.Passporter.com and the forums located within, you will find great information and tips from fellow vacationers that may assist you in planning.
We visited with a family member who is quite large (over 300 pounds) and there were no rides or shows that he was unable to enjoy. Some seats are more comfortable than others, of course, but all were do-able.
Disney is wonderfully accessible for a wide variety of guests of all shapes and sizes, and I know you will have a great vacation!