That's a toughie. I think as much consideration needs to be given for the size of the person as well as the chair and how mobile they are in the chair. Do they scoot along with their feet in the chair or actually use the handwheels? Or is someone else doing the pushing. I would say the less mobile they are, the larger you could go, otherwise I'd probably stick to a smaller lap size - maybe a tad larger than crib size. Don't know if that's much help for you, just some things to think about.