Our woodworkers club makes them for military funerals following this plan:
www.finewoodworkersofaustin.org/Plans/FlagCase.pdf
I made one for a neighbor following that plan. The flag was folded so tightly by the honor guard at the funeral that it felt like a rock, and was too small for the case. When refolded by mortals, still following the proper procedures, it fit the case fine.
Needless to say, there are many size flags, but I believe that most casket flags are 5 by 9 feet.