Now that FFG have gone big, the only way they can go is even bigger.
I think the closest precedent for size and scale we can have for the Death Star is the miniatures for the Borg Cube and Deep Space Nine releases for Star Trek Attack Wing.
It's definitely going to have to stretch the limits of the sliding scale, but it could be done. If the model has a large enough diameter its height will make it imposing and large enough over the other models on the board.