Wheel Chair Patient Measurement needed - Occupational Therapist

I know of a young man who hasn't any legs from the knee down. We are supplying him with a special "all terrain" wheel chair that is suitable for cobblestones, but need him measured so the chair will be the right size.

I'm wondering if there is any one around that knows how to make the measurement properly. I have the forms.

I did look around the Pemex on the Liebremento for the wheelchair/medical supply dealer but couldn't find it.