If I remember correctly, 45" square or 45 x 60 is what the alzheimers study (in San Diego area) was asking for in donated "lap quilts."
But I would ask the attendants at the nursing home. I'm sure they have some ideas. And the quilts probably get washed in hot water, and dried at high heat. Again, the nursing home attendants can probably verify this.
How nice of you to do this!