Usually people say 1/4 to 1/3 pound of meat per serving with 2 sides. Is it's for an annual banquet, 1/4 pound will probably be fine, but if it's after a full day of activity, you might need 1/2 pound each.
The way it is served will make a difference. Buffet style, you'll need more than if you have someone serving. What we sometimes do is have plenty of sides at the beginning of a buffet line, and at the end of the line, have someone portion out and serve the BBQ.
Good luck with it.