It really depends on the size of the person. I have seen recommendations of 36-45" square and several sizes for other rectangles. When I am in my chair, I prefer 45x54, which allows me to tuck my feet in nicely. An amputee, of course, will want a shorter quilt. Like Patchsamkin, I use the same size for baby quilts, on the theory that they can be crawled on early on and make nice TV quilts later.
PaperPrincess is right about labeling. 2 relatives had quilts "borrowed" in nursing homes. One was not returned ever.