i would ask someone here who is in a wheel chair as to the size..it would be nice to cover up while sitting. i have also heard that cutting off the bottom corners, or rounding them out, would help to keep them for getting caught in the wheels.
but i am sure someone here knows first hand..
what a wonderful thing to do!
this info was found at :
http://bcquilter.wordpress.com/quilt...n-quilt-sizes/
WHEELCHAIR LAP QUILT 36″ x 36″ to 38″ x 47″ or (91cm x 91cm to 97cm x 119cm ) adding ties is sometimes a good idea, so they dont slip off.
LAP between 52″ 68″ or 132cm x 173cm wide and the length can be from about 52″ 78″ or 132cm x 198cm. Although a lap quilt can be any size. A quilt that a child will want to bring along could be as small as 36″x36″ or 91cm x 91cm. A lap quilt that is 42″x 60″ 107cm x 152cm is great for cuddling up on the couch with.