I once did a king size on a 30 year old Kenmore. I rolled it really tight lengthwise and stipple quilted it. Quilting stores have clamps to help hold it.
You start in the middle and go towards the outside edge, then roll the other half and go towards the outside edge.
Now I have a baby lock and have done up to a queen size on it. Haven't needed to make a king size on it yet.