A standard king mattress is 76" wide x 80" long. Add 2x desired drop for width and 1x desired drop for length, plus a little allowance for sheets/blanket thickness.
On a 15" mattress, that makes it 106" wide x 95" long. I'd go 108" by 96"
On an 18" mattress, 112" wide x 98" long - I'd go 114" by 100"
n.b. if you're quilting this on a longarm, it will mostly likely need to be loaded sideways unless you have a 12 to 14' frame.