Janine,
We recently traveled with a family member who is quite large and I had these same concerns. I will say that the Passporter Open Mouse book was very helpful, so you might want to check that out. Additionally the
www.allearsnet.com website has a section called 'Disney at large' that you may find useful.
With my husband's uncle, we elected to rent an ECV from an independent company. This allowed him full use of the chair outside the parks as well, which was helpful at the resort. In researching I found that there are different styles of chairs for different sizes of people, so we were able to rent one that was designed to fit. Renting from a third party was also more economical than renting at the parks, and it gave us a guaranteed chair for use (the parks do have a limited number available and they do often run out). The company dropped it off at the resort so it was ready and waiting at check-in, and they picked it up after departure as well so there was absolutely no hassle at all for the renter.
I ran an internet search for 'Orlando Wheelchair Rental' to find a company. I've user Walker Mobility and Discount Mobility USA and was very pleased with both,