Sounds like you are talking about something shaped like the old orange floatation devices used in boats, but maybe with covering for the arms. Also like a cape. There was a free pattern for a cape a few years ago from Hancock Fabrics, designed to be used with fleece. It was basically a square with a hole cut out in the center with a little more cut out at the front of the neck, but there's no reason you couldn't cut it down the center from the neck area. The neck area would be a 16" circle. You'd probably want a square twice the length of the back of the neck to a chair surface of a person plus 8" (half the 16" neck cutout), I think that would be a 60" square (or 58" for a shorter person).