3/8 or 1/2 inch triangle would probably be fine. If the stab is still tight in the fuse it won't need bracing. For the broken area of stab see if you can relocate the pieces in a straight order and a little CA should hold. Think about putting doubles on anything that functions like a spar though. The entire elevator may need replacing. Just cut old CA hinges (if used) and re-slot for new ones near the old ones. Do NOT reuse old locations.