i use an 18" round plastic hoop. it might be pvc...i guess that would make sense.
i have a floor frame too....or sort of a guess. i haven't assembled it in years. i love the hand hoop because i am always quilting in one direction. i also know i won't be sitting in a gnarled up position for too long because i have to readjust the hoop from time to time
i keep my quilt moderately loose in the frame and make sure i take it off when i am finished for the session.
a king sized quilt is massive though....be prepared for the aching slowness of hand quilting. keep in mind it is heirloom quality work and keep on keepin' on.
aileen