How many people should we sit per table?

My sister and I are differing on the number to sit at the tables provided by the park venue. They are either 5 or 6 feet by 30 inches. We will be able to go check in the next day or two to be absolutly sure. So I looked up on a banquet site and they said that you can seat six people at a 6 foot table which is basically 3 on each side. My sister thinks we can also put one at each end making 8 at a table and serving will be buffet barbecue with nice heavy duty disposable plates. The guests will be in formal wear not picnic attire.

The venue is an old mill building with whitewashed walls and nice wood floors. The bride is our neice who's parents are both gone. she lives in NY and we and wedding are in dc.