Big enough if you want it as a quilt as it covers the top of the bed.
Not big enough, for the humps of people under that quilt ... without fighting!
Not big enough for a "bedspread" quilt ... as already mentioned by others.
What fits for others king beds, may or may not work for you.
A lot of variation on sizes according to .....
* mattress depth
* height of bed from floor
* do you want pillow tuck allowance?
* drape desired on sides
* is there a footboard to tuck into? or drape required there
There's probably some other factors, but this is a start.