The casserole will serve 20-24 people per full tray, the lesser if buffet, the greater with a server.

For the tuna salad, get the giant cans at a warehouse store or restaurant supply house. 66.5 ounces or just over 4 pounds. MUCH less expensive.

Here is a sample recipe to make over 100 sandwiches:

chunk light tuna in water, drained, flaked 8.25 lb. 1-1/2 gal. (2 large cans)
Mayonnaise 4.5 lb. 2-1/4 qt.
celery, finely diced 60 oz. 12 cups
sweet onions, diced 6 oz. 1-1/2 cups (may omit or use more)
red peppers, small dice 6 oz. 1-1/2 cups
curly-leaf parsley, chopped - 8 cups, 2 pounds
black pepper - 2 Tbsp.
celery salt - 1 tablespoon

You might look at the article on tea sandwiches for many other ideas and recipes: