The lap blankets/quilts the girls at church are....I believe....about 40x40. It really depends on how long your son's legs are when sitting down, and how wide his lap is.... You don't want it to drag on the floor, or get caught up in a wheel, but you still want it go feel cuddly.

What pattern and fabrics/colors will you be using?

I am so sorry that your son must use a wheel chair... I have a very good friend who has battled MS for years... She is also in a wheelchair now...

God Bless...