Will you be quilting this yourself? Otherwise, ask the person who will be quilting. If you have a 10 foot frame then 108 is about what I'd max out at width wise, and that's what I always consider king size -- 3 yards square.
For making it smaller, matching pillow shams can cover a bit of open space
We had a recent thread about extra wide backs, you might check out some of the sources here:
search for wide backing