Four adults and a child in a standard room would be too crowdy anyway if you ask me. Purely for size, I wouldn't book one of the most expensive resorts. I've done CBR (largest rooms of the mods) this last september, with a powerchair and manual. The room was large enough to get around in very comfortably in both chairs with the other still in the room. While I travelled alone, there was enough room for a second adult. I had a room with a roll in shower. This is something they'll need to think about, do they need a roll in shower? In that case he'ld be able to roll to the shower and toilet in his chair. In the other handicapped rooms, there are bars and a bath. Some prefer the first, some the latter.
I found the room to have some cooler spots, some warmer, some a bit drafty, some not drafty. There is enough shade and enough corners (for instance under table) for the dog to lie down, choosing whatever it would prefer itself. Also enough place to put a feedingbowl without standing/riding in it.
If they purely need a second room because of the disability (instead of 4, a disabled room will only fit 2 adults), special reservations will get them a second room for the remaining adults at no cost.
There are stickies on top of the disABILITIES section with info about the rooms and pics incl. sizes of all kinds of things. This can come in handy. If I remember correctly, the CBR rooms are even bigger than some of the Deluxe rooms, but not sure about that.
The dog will be welcome everywhere. DCLfun (kathy) will be able to tell you more about the practical side of it. I've seen her posting regularely about requesting an extra tight cleaning up after see leaves, so everybody knows for sure things are clean again and no doghairs are left behind in case anybody with an allergy for dogs would be next in that room. But again, the best info will come right out of Kathy's mouth.