I made one using a simple barn raising lay out of half-square triangles. Just kept it narrow. As I recall about 36 wide and maybe 40 long. This was made for a very tiny little lady so my size was adjusted for her.

I add ties approximately 4" in from the top edges. These can be used to tie the quilt to the arms of the wheelchair so quilt doesn't slide off the lap of the user. And I agree that it is a good idea to consult with the facility re: size.