Disney does a great job of accommodating the needs of guests with all
shapes, sizes and abilities. There are very few rides and attractions
at WDW that prove challenging
for plus-size people. Your mom should be able to experience almost
everything at Walt Disney World. To help you navigate I do have some
resources for you:
1. PassPorter's Open Mouse for Walt Disney World. This is the best
guidebook going for people with any sort of physical issue, including
size. Every ride at WDW is described in great detail, including the
size and type of seating and the type of restraint used on the ride.
This book is available through amazon.com and many larger bookstores.
2. The independent website allearsnet.com. Search there for a great
article called "WDW at Large" and also for "At Large" trip reports.
Lots of practical tips!
Additionally, you may find the following Moms Panel questions helpful:
HERE,
HERE, and
HERE.
Good luck with your planning.