I havent made a picnic table but, I did make a poker table a few years ago.
Point being, it has eight sides and is 53" across flat to flat. I wouldn't go much larger than that. A setting person can just reach the center at full reach. Can't have the catsup or napkins too far out of reach when you are hungry!
(or the chips when you win a hand
)
I post this because I know that it is too easy to think that "bigger is better" until you put something to use and find it is not at all practical.
Also, eight sides makes for good elbow room and seating. Seating can be anything from a chair or bench to even just a stool.
Rog