We just got back from WDW and I made sure to make note of the Companion Restrooms we used. As was already mentioned, some did not have a changing table. Some that don't have a changing table also don't have enough room to place a child on the floor on a towel (a specific one that didn't is the one near Pirates of the Caribbean at MK).
The ones that did have changing tables mostly had sort of a "shelf" between 40 and 48 inches long. Some were only about 36 inches long. The height was generally between 34 and 35 inches (which may be an issue if you have a heavier child to lift up there). Some had a towel dispensor mounted above one end of the counter (which might be an issue with being able to put a larger child up there).
We also found that we sometimes had a fairly long wait to use the Companion Restrooms. The longest waits were at MK - we had no choice there because the handicapped stalls at MK mostly don't have enough room for a wheelchair to for inside. We also found that every time we used the Companion Restroom, we had people try the door and knock at least once. So, if this is going to be a problem for you, you might want to be aware of it.