There are several recipes for beans for 100 on this site. It takes 2 roasters to make enough for 100, because you can actually only put 12-14 quarts in one roaster- otherwise the edges burn before the beans are done.
Green beans, 22 pounds per 100.
If you make ahead and reheat in roasters, preheat the roasters to 325 and allow 3 hours, turning down to 180 after they reach 165 in the middle of the pan.
I would do 16-18 pounds dry if I also made green beans, about 20-24 if it is really the only side dish, no potao salad, chips, etc.