They have one whole section blocked off for the Dinner Package, I would recommend sitting as close to center of the ampitheatre as possible. We didn't sit in the reserved section because we found better seats and only had to get there 30 minutes early!
The show was about 25 minutes long and it is incredible! One of my most favorite things to do at Disney! The seats do not have backs on them, but are like bleacher style seats.
Just came back from 8 days at the World, so if you have any questions, just ask!