Hi, Well i had events with 200 people, man get ready to smoke!
I always tend to cook over 200 people just in case. At an average sitting - an average american can eat 2 pounds worth of BBQ before feeling Buffet stuffed. (im 300 6'5 and 2 pounds is usually more than enough)
Portion wise, i would recommend the following per plate:
1 lb brisket
1 lb sausage
4oz beans
4oz salad.
brings you up to a total of 2.5lbs PER PLATE!!! this is more than enough for anyone + extras.
break down should be something similar to this. If you already know from your previous events - which proteins you prepare better, try to swing towards that meat protein a little more.
total breakdown
200 LB brisket
200 LB sausage
800oz Beans
800oz potato salad
hope this helped a bit
thanks