this bolt has broken twice on me. not the one that goes through the mounting brackey, but the part that connects to the alternator adjustment nut. i really need to replace those belts. anyway, here is a link to that part:
[i guess you have to click 300sd and then search for alternator - the link to the alternator adjustment bolt will come up.]
i don't know the exact measurements- it's probably about 6" long.