Check with the organization that you're sewing for - they may have specific requirements because of wheelchairs. I've seen them ranging from 37" x 48" on up to 45" x 55". If the quilts do not have a wheelchair restriction, lap quilts range from 52" x...
I've seen quilts go for $300-400 easy. And these are your traditional, random swatch type quilts. I would charge her as much as you think she will pay....that's a ton of work!!
You could try Amish Oak Direct at 611 Strasburg Rd, Paradise, PA 17562 tel no. (717) 687-8150 But this webiste is good if you want to locate any Amish made products.