Usually a boiled dinner is ham or corned beef but can also be beef round or pork roast, potatoes, carrots or rutabagas or parsnips or turnips, onions or leeks, cabbage. Sometimes a ring of kielbasa type sausage is added.
Here are the amounts I would use for 100:
65 pounds brisket or 55 pounds leaner meats
35 pounds potatoes
22-24 pounds carrot family
16-20 pounds boiler onions (you want 1 per person, depends on size)
18-20 pounds cabbage (small heads are better)
If adding sausage, 1 pound for each 5-6 people.
Use about 3/4 the amounts for 70.
To make this you simmer the meat until nearly tender, which can be 1 1/2 to 4 hours depending on type of meat. Usually the water is spiced with at least bay leaves or pickling spice.
Add the vegetables thus:
Cut an X in both ends of the peeled onions to prevent from splitting. Add peeled whole peeled onions to water during the final hour of cooking. Add baby carrots during the last 30 minutes of cooking. If using large carrots, slice into 3/8" coin slices and add during the final 40 minutes of cooking.
Quarter small cabbages, and add only during the last 40 minutes of cooking.
Add the potatoes the last 30-40 minutes, depending on the size of the chunks. The vegetables should NOT be mushy.
Mustard and horseradish/sourcream sauce are both common, and rye bread, of course.