So, a couple of fast questions:
- Are you staying in a WDW Resort Hotel?
- Are you planning on using the car for anything other than going to and from the parks and/or DTD-Disney Springs?
If you are staying on-property, and aren't planning on running around Orlando, then why not just take the Disney Magical Express from the airport? Depending on what your plans are, it may still be better to use Disney Magical Express and call a cab to drive you around Orlando.
We used to drive to WDW, because I am one of those poor souls who has to have her own car with her (I stink at carpooling LOL). The first time we flew in to Orlando, and took DME, I was very pleasantly surprised to find that I didn't really "miss" having a car. There is SO much to do at WDW that we never get it all done anyway...!
If you are staying off property, or have other things to do in Orlando, then I would try to rent an SUV or mini-van comparable to what you drive at home. It will not only reduce your stress level to drive a (somewhat) familiar vehicle, but it really does sound like you are going to need that extra space. Most 4-door sedans are simply not going to fit a wheelchair folded flat, and big pieces of luggage too. Trunk sizes are shrinking on cars as many manufacturers shift that space into the passenger cabin.
If you really need to rent a sedan, I would recommend going to a local car dealer, and looking at the model that you are going to rent. Take along a tape measure, and that will definitively answer your question about what will fit.