Hospital beds are twin size. But wouldn't you want something the person could take home and use? In that case I would make it at least double size. Plus, you'd want something to wrap up in - so again, double size. A double size quilt could be placed on top of a queen size bed without much trouble, although it might not be comfortable for two people in a queen sized bed to use, unless it were turned sideways.
Twin beds are 39", double 59", queen 65"? All beds are 78-80" long. Then you have to decide how much drop you want on each side. I would do a minimum of 12", probably 18".
So, if you really want to do a twin size, I'd do75" wide, 98" long
For double, I'd do minimum 84 x 98", better 96 x 104"