The event is for 225 and the main course (which I'm not responsible for) is steak and chicken. Here is what I'm planning to fill out the dinner:

Perfectly nice menu, but the amounts are off, and there is a basic principal of group feeding you want to consider- when offering a desirable item, have enough for everyone, even if you have to reduce the total number of items. In addition, no ovens if possible for beach picnics, eliminating the mac and cheese.

So, here are my suggestions:

Clam Bake - 50# clams, 25# potatoes, 75 ears corn
Linda, this is a very elaborate addition to the already fancy party, and will also run out, leaving many with clam or corn envy. If I were planning, I would do corn for everyone. If I wanted to add seafood, I would add cold boiled shrimp in appetizer portions (1 pound per about 6, 1 cup sauce per 8); must be served on ice for safety. Cooked ahead, chilled, much simpler.
Clam Chowder - 88 oz clams, 50# potatoes,
consider doing gazpacho (cold, doubles as a salad/veg) if it is a spring or summer party. Hot soup on the beach, not so easy to heat, serve, or eat... and you need at least 8-10 gallons
Potato Salad - 5 gallons- need 8-10
Bean Salad - 5 gallons- plenty
Egg Salad - 5 gallons- 3 plenty
Assorted Crackers- 12 pounds

In another post, you mention baked beans, about 7 gallons.

Fresh Fruit - 4# strawberries and grapes; 4 each cantaloupe, honeydew and watermelon
Way short- you need at least 1 1/2 times the amounts for the deluxe tray for 100 on the fruit tray page- or 4-6 "bites" per person

Mac and Cheese - 2 gallons- skip; you need an oven and would need at least 7 gallons for this party. Instead, do a big pasta salad, using 12 pounds dry pasta, make day ahead, keep cold (about 7-8 gallons)

Burgundy Mushrooms (to top steak) - 25#, plenty, if you are slicing
BBQ Sauce (to dip chicken) - 1 gallon? at least 1 cup per 8= 2 gallons

Sour Dough and French Bread - 6 baguettes; for 225 people, outside, you need at least 35 pounds of bread, 8 pounds of butter is nice.

Lemon Bars - 125; need 225 at least
Chocolate Brownies - 150; need 225 at least, about 3 bite size
Add 400 small cookies, I suggest oatmeal raisin and chocolate chip

Lemonade - frozen concentrate is good, about 18 gallons, but do have at least 6-8 gallons unsweetened ice tea for the no sugar folks
Bottled Water - 2 per person, 3 if long or hot party, buy from store where you can return unopened cases.