It varies a lot-- people are different sizes, and chairs are different sizes. I will disagree (respectfully) with the suggestion that she can just tuck it under her -- for people who have mobility issues, having a wad of quilt under them can cause pressure points or sores, in addition to being just plain uncomfortable. And quilts come untucked and drag or get caught in wheels or doors when they're too big.
So -- ask your aunt. Having it fit right and be usable trumps the fun of a surprise, if that was your intent. And if she doesn't know, either measure her while seated in her chair, or ask one of her caregivers to do so.
Failing that, measure yourself sitting in a chair. I'm not tiny, and a 36 inch wide by 42 long would cover my lap, come up a bit above my waist, and drape over my knees to my feet, but not cover my feet or get tangled when I go to stand up.