I haven't made a king size, yet that is...
I like the looks of a quilt hanging off of the bed, like a bedspread. It depends on the depth of the mattress and the height of the bed (some beds sit closer to the floor on their legs/platform), but for the most part it is a 21" drop from the flat of the mattress.
That said, I make all of my quilts, twin and queen sizes, the size of an old fashioned bedspread. They hang almost to the floor around the three sides (sides and bottom of bed) and there is enough to tuck in the pillows.
This way it is also long enough to snuggle under it and roll over without it falling off my shoulders or being tight.