I don't know the length, but if they sheared, they had to be loose. They don't rely on the bolt shaft to hold on the pulley, but rather the pinch action of the tightness of the bolt. I would get at least grade 10 on these/ I don't remember the length, but I think it is about 17mm or so.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS