Is the person thin or rounded?
If thin, I would guess the width of the mattress plus 10 inches (39 inch mattress plus 10 inches - 49-50 inches wide)
Length - My preference (I think) would be about 80 inches long (I sleep on my tummy - and hang my feet over the end of the bed - and like to be covered up to my ears)
If the person is very rounded, it might need to be wider.