I helped some kids make wheelchair quilts last year. If I remember correctly, we made the pocket about 16" wide and about 8" tall before sewing. We slanted the pockets like Carolyn (link in 1st reply) does. We lined the pocket with flannel, but didn't add the second pocket on the front. Carolyn also adds flannel to the quilt, so that the user has flannel on both sides of the pocket. We just used the "birthing" method for the pocket and sewed it on the front.
If you'll line the quilt with flannel, it won't slide off the user's lap and you won't need to tie it on. We also clipped the corners of the bottom, so that the corners do not get caught in the wheels of the chair.
bkay