I'd be concerned about the height that you would need to lift the wheelchair to get it in to the back. It may not be a problem if the child's power wheelchair isn't that heavy, but my DD's is a beast.
I think the easiest might be to go to a dealer and check out the actual size compared to your child's wheelchair. One time when we bought a new vehicle, we thought we would be able to get DD's manual wheelchair into the back from the dimensions we found on the car specs. but once we actually saw it, we could tell right away the wheelchair would not fit in the space because of the angle of the back liftgate.