All WDW onsite resorts have accessible rooms throughout the resort with various views and building locations. I'm not sure that one resort is any more "handicapped friendly" than another, but there likely could be some aspects causing one to prefer a certain resort. I'd suggest looking at the various resorts and deciding if there is a theme or location that you prefer (or she might prefer). Also, which category of deluxe or moderate or value fits the budget, as that can be a huge deciding factor. You might want to consider the transportation options unless you will have a vehicle and plan to drive to the parks.
Most rooms with a roll-in shower have a single king-size bed. There are a limited number of king+double rooms available at each resort; I believe that may be quite a limited number, so booking early gives you the best choice of room options. Deluxe resorts have larger rooms, value resorts the smallest, but any will have enough space for her personal
scooter. Be aware of WDW's terminology regarding "adjoining" rooms -- adjoining means "near" (possibly next to but could be down the hall) and "connecting" has an interior door allowing access between rooms. Requesting connecting or adjoining rooms is just a request and can't be guaranteed, though WDW does their best to honor such requests whenever possible.
Enjoy your vacation!