I don't have a wheelchair to measure, but I just measured me in my desk chair on wheels, and I'm guessing that may be a bit too large. I'm thinking maybe 30" x 50" would be plenty large enough. You don't want excess on the sides to catch in the wheels there, nor do you want it so long that it drags and gets rolled over, right? A length of 50" should be long enough to pull up over his chest if he wants, I think. If you want it to just cover his lap and legs you'd want it shorter. The best indicator is if someone could measure over themselves or someone else sitting in a wheelchair and le you know. HTH