I just finished one that was 86 x 108 inches. It is something I would do again, but isn't necessary since everybody else has full-sized beds, not queen or king.
I took a suggestion from another quilter and only worked on it an hour at a time. The weight of the quilt got exhausting, even with an extra table behind machine, and then I would make mistakes. It got done and I'm thrilled with it.
Most of my quilts are between 64 and 80 inches wide by 96 inches long. I just bought a Bernina 440QE this month and it has an extra 2 inches between the needle and the motor housing, which makes quilting a large quilt quite a bit easier, surprisingly. Still, if I had to do a King size, I would do that section by section and then assemble the sections. I can't afford to send more than one quilt out to be quilted annually. I tend to spend my budget in advance on fabric!